{"id":565248,"date":"2026-07-10T09:40:05","date_gmt":"2026-07-10T02:40:05","guid":{"rendered":"https:\/\/hoanghamobile.com\/tin-tuc\/?p=565248"},"modified":"2026-07-10T09:40:20","modified_gmt":"2026-07-10T02:40:20","slug":"dinh-ly-cosin","status":"publish","type":"post","link":"https:\/\/hoanghamobile.com\/tin-tuc\/dinh-ly-cosin\/","title":{"rendered":"\u0110\u1ecbnh l\u00fd cosin l\u00e0 g\u00ec? C\u00f4ng th\u1ee9c, ch\u1ee9ng minh v\u00e0 c\u00e1ch \u00e1p d\u1ee5ng chi ti\u1ebft"},"content":{"rendered":"<p style=\"text-align: justify;\"><b>\u0110\u1ecbnh l\u00fd cosin<\/b><span style=\"font-weight: 400;\"> l\u00e0 c\u00f4ng th\u1ee9c li\u00ean h\u1ec7 gi\u1eefa ba c\u1ea1nh v\u00e0 m\u1ed9t g\u00f3c c\u1ee7a tam gi\u00e1c: b\u00ecnh ph\u01b0\u01a1ng m\u1ed9t c\u1ea1nh b\u1eb1ng t\u1ed5ng b\u00ecnh ph\u01b0\u01a1ng hai c\u1ea1nh c\u00f2n l\u1ea1i tr\u1eeb \u0111i hai l\u1ea7n t\u00edch hai c\u1ea1nh \u0111\u00f3 nh\u00e2n cosin g\u00f3c xen gi\u1eefa. C\u00f4ng th\u1ee9c t\u1ed5ng qu\u00e1t: a\u00b2 = b\u00b2 + c\u00b2 \u2212 2bc\u00b7cosA. \u0110\u00e2y l\u00e0 ki\u1ebfn th\u1ee9c tr\u1ecdng t\u00e2m trong ch\u01b0\u01a1ng tr\u00ecnh To\u00e1n l\u1edbp 10, d\u00f9ng \u0111\u1ec3 t\u00ednh c\u1ea1nh ho\u1eb7c g\u00f3c khi tam gi\u00e1c kh\u00f4ng c\u00f3 g\u00f3c vu\u00f4ng. B\u00e0i vi\u1ebft s\u1ebd gi\u00fap b\u1ea1n n\u1eafm ch\u1eafc c\u00f4ng th\u1ee9c, c\u00e1ch ch\u1ee9ng minh v\u00e0 \u00e1p d\u1ee5ng qua v\u00ed d\u1ee5 c\u00f3 l\u1eddi gi\u1ea3i chi ti\u1ebft.<\/span><\/p>\n<h2 style=\"text-align: justify;\"><b>\u0110\u1ecbnh l\u00fd cosin l\u00e0 g\u00ec?<\/b><\/h2>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Trong tam gi\u00e1c ABC, v\u1edbi c\u00e1c c\u1ea1nh a, b, c l\u1ea7n l\u01b0\u1ee3t \u0111\u1ed1i di\u1ec7n v\u1edbi c\u00e1c g\u00f3c A, B, C, \u0111\u1ecbnh l\u00fd cosin m\u00f4 t\u1ea3 m\u1ed1i quan h\u1ec7 gi\u1eefa \u0111\u1ed9 d\u00e0i ba c\u1ea1nh v\u00e0 s\u1ed1 \u0111o c\u00e1c g\u00f3c trong tam gi\u00e1c. Theo \u0111\u1ecbnh l\u00fd n\u00e0y, b\u00ecnh ph\u01b0\u01a1ng \u0111\u1ed9 d\u00e0i c\u1ee7a m\u1ed9t c\u1ea1nh b\u1eb1ng t\u1ed5ng b\u00ecnh ph\u01b0\u01a1ng \u0111\u1ed9 d\u00e0i c\u1ee7a hai c\u1ea1nh c\u00f2n l\u1ea1i, tr\u1eeb \u0111i hai l\u1ea7n t\u00edch c\u1ee7a hai c\u1ea1nh \u0111\u00f3 nh\u00e2n v\u1edbi cosin c\u1ee7a g\u00f3c xen gi\u1eefa ch\u00fang.<\/span><\/p>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"aligncenter size-full wp-image-565252\" src=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-1.jpg\" alt=\"dinh-ly-cosin-1\" width=\"800\" height=\"450\" srcset=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-1.jpg 800w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-1-300x169.jpg 300w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-1-768x432.jpg 768w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">C\u00f4ng th\u1ee9c cosin \u0111\u01b0\u1ee3c xem l\u00e0 s\u1ef1 m\u1edf r\u1ed9ng c\u1ee7a <\/span><b>\u0111\u1ecbnh l\u00fd Pythagore<\/b><span style=\"font-weight: 400;\">, b\u1edfi khi g\u00f3c xen gi\u1eefa hai c\u1ea1nh b\u1eb1ng 90\u00b0, gi\u00e1 tr\u1ecb cos 90\u00b0 = 0 n\u00ean c\u00f4ng th\u1ee9c cosin tr\u1edf v\u1ec1 d\u1ea1ng quen thu\u1ed9c: a\u00b2 = b\u00b2 + c\u00b2. Tuy nhi\u00ean, kh\u00e1c v\u1edbi \u0111\u1ecbnh l\u00fd Pythagore ch\u1ec9 \u00e1p d\u1ee5ng cho tam gi\u00e1c vu\u00f4ng, \u0111\u1ecbnh l\u00fd cosin c\u00f3 th\u1ec3 s\u1eed d\u1ee5ng trong m\u1ecdi lo\u1ea1i tam gi\u00e1c, bao g\u1ed3m tam gi\u00e1c nh\u1ecdn, tam gi\u00e1c t\u00f9 v\u00e0 tam gi\u00e1c vu\u00f4ng.<\/span><\/p>\n<h2 style=\"text-align: justify;\"><b>C\u00f4ng th\u1ee9c \u0111\u1ecbnh l\u00fd cosin<\/b><\/h2>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">V\u1edbi tam gi\u00e1c ABC b\u1ea5t k\u1ef3, \u0111\u1ecbnh l\u00fd cosin c\u00f3 ba d\u1ea1ng c\u00f4ng th\u1ee9c t\u01b0\u01a1ng \u1ee9ng v\u1edbi ba c\u1ea1nh. B\u1ea1n ch\u1ec9 c\u1ea7n nh\u1edb m\u1ed9t d\u1ea1ng r\u1ed3i ho\u00e1n v\u1ecb c\u00e1c c\u1ea1nh v\u00e0 g\u00f3c l\u00e0 suy ra hai d\u1ea1ng c\u00f2n l\u1ea1i.<\/span><\/p>\n<ul style=\"text-align: justify;\">\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">a\u00b2 = b\u00b2 + c\u00b2 \u2212 2bc\u00b7cosA<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">b\u00b2 = a\u00b2 + c\u00b2 \u2212 2ac\u00b7cosB<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">c\u00b2 = a\u00b2 + b\u00b2 \u2212 2ab\u00b7cosC<\/span><\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">T\u1eeb ba c\u00f4ng th\u1ee9c tr\u00ean, c\u00f3 th\u1ec3 suy ra h\u1ec7 qu\u1ea3 \u0111\u1ecbnh l\u00fd cosin \u0111\u1ec3 t\u00ednh g\u00f3c khi bi\u1ebft \u0111\u1ed9 d\u00e0i ba c\u1ea1nh:<\/span><\/p>\n<ul style=\"text-align: justify;\">\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">cosA = (b\u00b2 + c\u00b2 \u2212 a\u00b2) \/ (2bc)<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">cosB = (a\u00b2 + c\u00b2 \u2212 b\u00b2) \/ (2ac)<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">cosC = (a\u00b2 + b\u00b2 \u2212 c\u00b2) \/ (2ab)<\/span><\/li>\n<\/ul>\n<h2 style=\"text-align: justify;\"><b>Ch\u1ee9ng minh \u0111\u1ecbnh l\u00fd cosin<\/b><\/h2>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">C\u00e1ch ch\u1ee9ng minh ph\u1ed5 bi\u1ebfn v\u00e0 ng\u1eafn g\u1ecdn nh\u1ea5t l\u00e0 d\u00f9ng vect\u01a1, d\u1ef1a tr\u00ean t\u00ednh ch\u1ea5t c\u1ee7a t\u00edch v\u00f4 h\u01b0\u1edbng.<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Ch\u1ee9ng minh b\u1eb1ng vect\u01a1: X\u00e9t tam gi\u00e1c ABC, ta c\u00f3 (d\u01b0\u1edbi d\u1ea1ng vect\u01a1): $\\vec{BC} = \\vec{AC} \u2212 \\vec{AB}$.<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">B\u00ecnh ph\u01b0\u01a1ng \u0111\u1ed9 d\u00e0i vect\u01a1 BC:<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">BC\u00b2 = (AC \u2212 AB)\u00b2 = AC\u00b2 + AB\u00b2 \u2212 2\u00b7(AC\u00b7AB)<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Trong \u0111\u00f3 t\u00edch v\u00f4 h\u01b0\u1edbng AC\u00b7AB = |AC| \u00d7 |AB| \u00d7 cosA. Thay AC = b, AB = c, BC = a, ta \u0111\u01b0\u1ee3c:<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">a\u00b2 = b\u00b2 + c\u00b2 \u2212 2bc\u00b7cosA (\u0111i\u1ec1u ph\u1ea3i ch\u1ee9ng minh).<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Ki\u1ec3m ch\u1ee9ng v\u1edbi tam gi\u00e1c vu\u00f4ng: n\u1ebfu g\u00f3c A = 90\u00b0 th\u00ec cosA = 0, c\u00f4ng th\u1ee9c tr\u1edf th\u00e0nh a\u00b2 = b\u00b2 + c\u00b2, ch\u00ednh l\u00e0 \u0111\u1ecbnh l\u00fd Pythagore. \u0110i\u1ec1u n\u00e0y kh\u1eb3ng \u0111\u1ecbnh \u0111\u1ecbnh l\u00fd cosin l\u00e0 tr\u01b0\u1eddng h\u1ee3p t\u1ed5ng qu\u00e1t h\u00f3a c\u1ee7a \u0111\u1ecbnh l\u00fd Pythagore.<\/span><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-full wp-image-565251\" src=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-2.jpg\" alt=\"dinh-ly-cosin-2\" width=\"800\" height=\"450\" srcset=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-2.jpg 800w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-2-300x169.jpg 300w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-2-768x432.jpg 768w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/p>\n<h2 style=\"text-align: justify;\"><b>C\u00e1ch \u00e1p d\u1ee5ng \u0111\u1ecbnh l\u00fd cosin<\/b><\/h2>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">C\u00e1ch \u00e1p d\u1ee5ng \u0111\u1ecbnh l\u00fd cosin gi\u00fap b\u1ea1n gi\u1ea3i nhanh c\u00e1c b\u00e0i to\u00e1n tam gi\u00e1c b\u1eb1ng c\u00e1ch x\u00e1c \u0111\u1ecbnh c\u1ea1nh ho\u1eb7c g\u00f3c ch\u01b0a bi\u1ebft d\u1ef1a tr\u00ean d\u1eef ki\u1ec7n \u0111\u00e3 cho. T\u00f9y t\u1eebng tr\u01b0\u1eddng h\u1ee3p, \u0111\u1ecbnh l\u00fd c\u00f3 th\u1ec3 d\u00f9ng \u0111\u1ec3 t\u00ednh c\u1ea1nh khi bi\u1ebft hai c\u1ea1nh v\u00e0 g\u00f3c xen gi\u1eefa, ho\u1eb7c t\u00ednh g\u00f3c khi bi\u1ebft \u0111\u1ed9 d\u00e0i ba c\u1ea1nh. N\u1eafm v\u1eefng c\u00e1ch nh\u1eadn bi\u1ebft v\u00e0 l\u1ef1a ch\u1ecdn c\u00f4ng th\u1ee9c ph\u00f9 h\u1ee3p s\u1ebd gi\u00fap vi\u1ec7c gi\u1ea3i b\u00e0i t\u1eadp h\u00ecnh h\u1ecdc l\u1edbp 10 tr\u1edf n\u00ean \u0111\u01a1n gi\u1ea3n v\u00e0 ch\u00ednh x\u00e1c h\u01a1n.<\/span><\/p>\n<h3 style=\"text-align: justify;\">T\u00ednh c\u1ea1nh tam gi\u00e1c<\/h3>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">V\u00ed d\u1ee5: Tam gi\u00e1c ABC c\u00f3 b = 8 cm, c = 5 cm, g\u00f3c A = 60\u00b0. T\u00ednh c\u1ea1nh a. C\u00e1c b\u01b0\u1edbc thay s\u1ed1 v\u00e0 t\u00ednh to\u00e1n \u0111\u01b0\u1ee3c tr\u00ecnh b\u00e0y trong b\u1ea3ng d\u01b0\u1edbi \u0111\u00e2y.<\/span><\/p>\n<table>\n<thead>\n<tr>\n<th>B\u01b0\u1edbc<\/th>\n<th>Ph\u00e9p t\u00ednh<\/th>\n<th>K\u1ebft qu\u1ea3<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>\u00c1p d\u1ee5ng c\u00f4ng th\u1ee9c<\/td>\n<td>a\u00b2 = b\u00b2 + c\u00b2 \u2212 2bc\u00b7cosA<\/td>\n<td>\u2014<\/td>\n<\/tr>\n<tr>\n<td>Thay s\u1ed1<\/td>\n<td>a\u00b2 = 8\u00b2 + 5\u00b2 \u2212 2\u00d78\u00d75\u00d7cos60\u00b0<\/td>\n<td>a\u00b2 = 64 + 25 \u2212 80\u00d70,5<\/td>\n<\/tr>\n<tr>\n<td>R\u00fat g\u1ecdn<\/td>\n<td>a\u00b2 = 89 \u2212 40<\/td>\n<td>a\u00b2 = 49<\/td>\n<\/tr>\n<tr>\n<td>L\u1ea5y c\u0103n b\u1eadc hai<\/td>\n<td>a = \u221a49<\/td>\n<td><strong>a = 7 cm<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3 style=\"text-align: justify;\"><b>T\u00ednh g\u00f3c b\u1eb1ng \u0111\u1ecbnh l\u00fd cosin<\/b><\/h3>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">V\u00ed d\u1ee5: Tam gi\u00e1c ABC c\u00f3 a = 7, b = 8, c = 5. T\u00ednh g\u00f3c A b\u1eb1ng h\u1ec7 qu\u1ea3 \u0111\u1ecbnh l\u00fd cosin.<\/span><\/p>\n<table>\n<thead>\n<tr>\n<th>B\u01b0\u1edbc<\/th>\n<th>Ph\u00e9p t\u00ednh<\/th>\n<th>K\u1ebft qu\u1ea3<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>\u00c1p d\u1ee5ng h\u1ec7 qu\u1ea3<\/td>\n<td>cosA = (b\u00b2 + c\u00b2 \u2212 a\u00b2) \/ (2bc)<\/td>\n<td>\u2014<\/td>\n<\/tr>\n<tr>\n<td>Thay s\u1ed1<\/td>\n<td>cosA = (64 + 25 \u2212 49) \/ (2\u00d78\u00d75)<\/td>\n<td>cosA = 40 \/ 80<\/td>\n<\/tr>\n<tr>\n<td>R\u00fat g\u1ecdn<\/td>\n<td>cosA = 0,5<\/td>\n<td>\u2014<\/td>\n<\/tr>\n<tr>\n<td>Suy ra g\u00f3c<\/td>\n<td>A = arccos(0,5)<\/td>\n<td><strong>A = 60\u00b0<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3 style=\"text-align: justify;\"><b>C\u00e1c b\u01b0\u1edbc gi\u1ea3i nhanh m\u1ed9t b\u00e0i to\u00e1n cosin<\/b><\/h3>\n<ul style=\"text-align: justify;\">\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">X\u00e1c \u0111\u1ecbnh d\u1eef ki\u1ec7n \u0111\u1ec1 b\u00e0i cho (2 c\u1ea1nh + g\u00f3c xen gi\u1eefa, hay 3 c\u1ea1nh).<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Ch\u1ecdn \u0111\u00fang c\u00f4ng th\u1ee9c t\u01b0\u01a1ng \u1ee9ng (t\u00ednh c\u1ea1nh hay t\u00ednh g\u00f3c).<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">Thay s\u1ed1 c\u1ea9n th\u1eadn, ch\u00fa \u00fd \u0111\u01a1n v\u1ecb g\u00f3c (\u0111\u1ed9 ho\u1eb7c radian).<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">T\u00ednh to\u00e1n v\u00e0 ki\u1ec3m tra l\u1ea1i k\u1ebft qu\u1ea3 b\u1eb1ng b\u1ea5t \u0111\u1eb3ng th\u1ee9c tam gi\u00e1c.<\/span><\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Khi th\u1ef1c h\u00e0nh, b\u1ea1n n\u00ean b\u1ea5m m\u00e1y t\u00ednh c\u1ea7m tay c\u00f3 \u0111\u1ea7y \u0111\u1ee7 h\u00e0m l\u01b0\u1ee3ng gi\u00e1c (ch\u1eb3ng h\u1ea1n d\u00f2ng Casio FX-580VN X \u0111\u01b0\u1ee3c ph\u00e9p mang v\u00e0o ph\u00f2ng thi THPT Qu\u1ed1c gia) v\u00e0 lu\u00f4n \u0111\u1ec3 m\u00e1y \u1edf ch\u1ebf \u0111\u1ed9 Deg khi \u0111\u1ec1 cho g\u00f3c theo \u0111\u01a1n v\u1ecb \u0111\u1ed9, tr\u00e1nh sai s\u1ed1 do nh\u1ea7m sang radian.<\/span><\/p>\n<p><img decoding=\"async\" class=\"aligncenter size-full wp-image-565250\" src=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-3.jpg\" alt=\"dinh-ly-cosin-3\" width=\"800\" height=\"450\" srcset=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-3.jpg 800w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-3-300x169.jpg 300w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-3-768x432.jpg 768w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/p>\n<h2 style=\"text-align: justify;\"><b>\u0110\u1ecbnh l\u00fd sin v\u00e0 \u0111\u1ecbnh l\u00fd cosin: khi n\u00e0o d\u00f9ng c\u00e1i n\u00e0o?<\/b><\/h2>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Hai \u0111\u1ecbnh l\u00fd n\u00e0y b\u1ed5 sung cho nhau khi gi\u1ea3i tam gi\u00e1c. Vi\u1ec7c ch\u1ecdn \u0111\u00fang \u0111\u1ecbnh l\u00fd ngay t\u1eeb \u0111\u1ea7u s\u1ebd gi\u00fap b\u1ea1n ti\u1ebft ki\u1ec7m th\u1eddi gian v\u00e0 tr\u00e1nh v\u00f2ng vo.<\/span><\/p>\n<ul style=\"text-align: justify;\">\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">D\u00f9ng \u0111\u1ecbnh l\u00fd cosin khi: bi\u1ebft 2 c\u1ea1nh + g\u00f3c xen gi\u1eefa (c-g-c), ho\u1eb7c bi\u1ebft c\u1ea3 3 c\u1ea1nh (c-c-c).<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">D\u00f9ng \u0111\u1ecbnh l\u00fd sin khi: bi\u1ebft 1 c\u1ea1nh + 2 g\u00f3c (g-c-g), ho\u1eb7c bi\u1ebft 2 c\u1ea1nh + 1 g\u00f3c \u0111\u1ed1i di\u1ec7n (t\u00ecm g\u00f3c c\u00f2n l\u1ea1i).<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">C\u00f4ng th\u1ee9c \u0111\u1ecbnh l\u00fd sin: a\/sinA = b\/sinB = c\/sinC = 2R (R l\u00e0 b\u00e1n k\u00ednh \u0111\u01b0\u1eddng tr\u00f2n ngo\u1ea1i ti\u1ebfp).<\/span><\/li>\n<\/ul>\n<p style=\"text-align: justify;\">N\u1ebfu ch\u01b0a n\u1eafm ch\u1eafc b\u1ea3n ch\u1ea5t c\u00e1c t\u1ef7 s\u1ed1 l\u01b0\u1ee3ng gi\u00e1c, b\u1ea1n n\u00ean xem l\u1ea1i <a href=\"https:\/\/hoanghamobile.com\/tin-tuc\/sin-cos-tan-la-gi\/\"><span style=\"font-weight: 400;\">Sin Cos Tan l\u00e0 g\u00ec<\/span><\/a> tr\u01b0\u1edbc khi h\u1ecdc s\u00e2u v\u1ec1 \u0111\u1ecbnh l\u00fd sin v\u00e0 cosin. Trong nhi\u1ec1u b\u00e0i to\u00e1n, h\u1ecdc sinh c\u1ea7n ph\u1ed1i h\u1ee3p c\u1ea3 hai \u0111\u1ecbnh l\u00fd: d\u00f9ng cosin \u0111\u1ec3 t\u00ecm c\u1ea1nh ho\u1eb7c g\u00f3c c\u00f2n thi\u1ebfu, sau \u0111\u00f3 d\u00f9ng sin \u0111\u1ec3 t\u00ednh c\u00e1c y\u1ebfu t\u1ed1 c\u00f2n l\u1ea1i ho\u1eb7c di\u1ec7n t\u00edch tam gi\u00e1c.<\/p>\n<h2 style=\"text-align: justify;\"><b>B\u00e0i t\u1eadp \u0111\u1ecbnh l\u00fd cosin c\u00f3 l\u1eddi gi\u1ea3i<\/b><\/h2>\n<p style=\"text-align: justify;\"><b>B\u00e0i 1:<\/b><span style=\"font-weight: 400;\"> Tam gi\u00e1c ABC c\u00f3 a = 6, b = 7, g\u00f3c C = 45\u00b0. T\u00ednh c\u1ea1nh c.<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Gi\u1ea3i: c\u00b2 = a\u00b2 + b\u00b2 \u2212 2ab\u00b7cosC = 6\u00b2 + 7\u00b2 \u2212 2\u00d76\u00d77\u00d7cos45\u00b0 = 36 + 49 \u2212 84\u00d70,707 \u2248 85 \u2212 59,4 \u2248 25,6. Suy ra c \u2248 5,06.<\/span><\/p>\n<p style=\"text-align: justify;\"><b>B\u00e0i 2:<\/b><span style=\"font-weight: 400;\"> Tam gi\u00e1c c\u00f3 ba c\u1ea1nh a = 5, b = 6, c = 7. T\u00ednh g\u00f3c l\u1edbn nh\u1ea5t c\u1ee7a tam gi\u00e1c.<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Gi\u1ea3i: G\u00f3c l\u1edbn nh\u1ea5t \u0111\u1ed1i di\u1ec7n c\u1ea1nh d\u00e0i nh\u1ea5t (c = 7), n\u00ean ta t\u00ednh g\u00f3c C.<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">cosC = (a\u00b2 + b\u00b2 \u2212 c\u00b2)\/(2ab) = (25 + 36 \u2212 49)\/(2\u00d75\u00d76) = 12\/60 = 0,2. Suy ra g\u00f3c C \u2248 78,46\u00b0.<\/span><\/p>\n<p style=\"text-align: justify;\"><b>B\u00e0i 3:<\/b><span style=\"font-weight: 400;\"> T\u1eeb m\u1ed9t \u0111i\u1ec3m quan s\u00e1t, ng\u01b0\u1eddi ta \u0111o \u0111\u01b0\u1ee3c hai v\u1ecb tr\u00ed A v\u00e0 B c\u00e1ch nhau 50 m v\u00e0 c\u00f9ng nh\u00ecn v\u1ec1 c\u1ed9t c\u1edd C. Bi\u1ebft g\u00f3c CAB = 40\u00b0, kho\u1ea3ng c\u00e1ch AC = 70 m. T\u00ednh kho\u1ea3ng c\u00e1ch BC.<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Gi\u1ea3i: \u00c1p d\u1ee5ng \u0111\u1ecbnh l\u00fd cosin trong tam gi\u00e1c ABC:<\/span><\/p>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">BC\u00b2 = AB\u00b2 + AC\u00b2 \u2212 2\u00d7AB\u00d7AC\u00d7cosA = 50\u00b2 + 70\u00b2 \u2212 2\u00d750\u00d770\u00d7cos40\u00b0 = 2500 + 4900 \u2212 7000\u00d70,766 \u2248 7400 \u2212 5362 = 2038. Suy ra BC \u2248 45,1 m.<\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-565253\" src=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-4.jpg\" alt=\"dinh-ly-cosin-4\" width=\"800\" height=\"450\" srcset=\"https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-4.jpg 800w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-4-300x169.jpg 300w, https:\/\/hoanghamobile.com\/tin-tuc\/wp-content\/uploads\/2026\/07\/dinh-ly-cosin-4-768x432.jpg 768w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/p>\n<h2 style=\"text-align: justify;\"><b>\u1ee8ng d\u1ee5ng th\u1ef1c t\u1ebf c\u1ee7a \u0111\u1ecbnh l\u00fd cosin<\/b><\/h2>\n<p style=\"text-align: justify;\"><span style=\"font-weight: 400;\">Ngo\u00e0i ph\u00f2ng thi, \u0111\u1ecbnh l\u00fd cosin c\u00f2n xu\u1ea5t hi\u1ec7n trong nhi\u1ec1u l\u0129nh v\u1ef1c k\u1ef9 thu\u1eadt v\u00e0 \u0111\u1eddi s\u1ed1ng, n\u01a1i c\u1ea7n t\u00ednh kho\u1ea3ng c\u00e1ch ho\u1eb7c g\u00f3c m\u00e0 kh\u00f4ng th\u1ec3 \u0111o tr\u1ef1c ti\u1ebfp.<\/span><\/p>\n<ul style=\"text-align: justify;\">\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">\u0110o \u0111\u1ea1c, kh\u1ea3o s\u00e1t \u0111\u1ecba h\u00ecnh: t\u00ednh kho\u1ea3ng c\u00e1ch gi\u00e1n ti\u1ebfp gi\u1eefa hai \u0111i\u1ec3m kh\u00f4ng th\u1ec3 \u0111o tr\u1ef1c ti\u1ebfp.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">H\u00e0ng h\u1ea3i, h\u00e0ng kh\u00f4ng: x\u00e1c \u0111\u1ecbnh v\u1ecb tr\u00ed, kho\u1ea3ng c\u00e1ch gi\u1eefa t\u00e0u ho\u1eb7c m\u00e1y bay v\u1edbi c\u00e1c \u0111i\u1ec3m m\u1ed1c.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">\u0110\u1ecbnh v\u1ecb GPS: t\u00ednh to\u00e1n trong h\u1ec7 th\u1ed1ng tam gi\u00e1c \u0111\u1ea1c \u0111\u1ec3 x\u00e1c \u0111\u1ecbnh t\u1ecda \u0111\u1ed9.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">K\u1ef9 thu\u1eadt x\u00e2y d\u1ef1ng: t\u00ednh to\u00e1n k\u1ebft c\u1ea5u, g\u00f3c nghi\u00eang trong thi\u1ebft k\u1ebf c\u00f4ng tr\u00ecnh.<\/span><\/li>\n<li style=\"font-weight: 400;\" aria-level=\"1\"><span style=\"font-weight: 400;\">V\u1eadt l\u00fd: t\u00ednh h\u1ee3p l\u1ef1c khi bi\u1ebft \u0111\u1ed9 l\u1edbn hai l\u1ef1c v\u00e0 g\u00f3c xen gi\u1eefa (c\u00f4ng th\u1ee9c t\u01b0\u01a1ng t\u1ef1).<\/span><\/li>\n<\/ul>\n<h2 style=\"text-align: justify;\"><b>C\u00e2u h\u1ecfi th\u01b0\u1eddng g\u1eb7p<\/b><\/h2>\n<h3 style=\"text-align: justify;\"><strong>\u0110\u1ecbnh l\u00fd cosin d\u00f9ng khi n\u00e0o?<\/strong><\/h3>\n<p style=\"text-align: justify;\">\u0110\u1ecbnh l\u00fd cosin d\u00f9ng khi bi\u1ebft 2 c\u1ea1nh v\u00e0 g\u00f3c xen gi\u1eefa \u0111\u1ec3 t\u00ednh c\u1ea1nh c\u00f2n l\u1ea1i, ho\u1eb7c bi\u1ebft c\u1ea3 3 c\u1ea1nh \u0111\u1ec3 t\u00ednh m\u1ed9t g\u00f3c b\u1ea5t k\u1ef3 c\u1ee7a tam gi\u00e1c.<\/p>\n<h3 style=\"text-align: justify;\"><strong>\u0110\u1ecbnh l\u00fd cosin kh\u00e1c \u0111\u1ecbnh l\u00fd Pythagore \u1edf \u0111i\u1ec3m n\u00e0o?<\/strong><\/h3>\n<p style=\"text-align: justify;\">\u0110\u1ecbnh l\u00fd Pythagore ch\u1ec9 \u00e1p d\u1ee5ng cho tam gi\u00e1c vu\u00f4ng (a\u00b2 = b\u00b2 + c\u00b2), c\u00f2n \u0111\u1ecbnh l\u00fd cosin \u00e1p d\u1ee5ng cho m\u1ecdi tam gi\u00e1c v\u00e0 c\u00f3 th\u00eam s\u1ed1 h\u1ea1ng \u22122bc\u00b7cosA. Khi g\u00f3c A = 90\u00b0, c\u00f4ng th\u1ee9c thu v\u1ec1 \u0111\u00fang \u0111\u1ecbnh l\u00fd Pythagore.<\/p>\n<h3 style=\"text-align: justify;\"><strong>L\u00e0m sao nh\u1edb c\u00f4ng th\u1ee9c \u0111\u1ecbnh l\u00fd cosin nhanh?<\/strong><\/h3>\n<p style=\"text-align: justify;\">B\u1ea1n c\u00f3 th\u1ec3 nh\u1edb theo m\u1eabu: &#8220;b\u00ecnh ph\u01b0\u01a1ng c\u1ea1nh \u0111\u1ed1i = t\u1ed5ng b\u00ecnh ph\u01b0\u01a1ng hai c\u1ea1nh k\u1ec1 \u2212 2 l\u1ea7n t\u00edch hai c\u1ea1nh k\u1ec1 nh\u00e2n cosin g\u00f3c xen gi\u1eefa&#8221;. C\u1ea1nh \u0111\u1ee9ng m\u1ed9t m\u00ecnh b\u00ean tr\u00e1i lu\u00f4n l\u00e0 c\u1ea1nh \u0111\u1ed1i di\u1ec7n v\u1edbi g\u00f3c xu\u1ea5t hi\u1ec7n trong cosin.<\/p>\n<h3 style=\"text-align: justify;\"><strong>\u0110\u1ecbnh l\u00fd cosin c\u00f3 \u00e1p d\u1ee5ng \u0111\u01b0\u1ee3c cho tam gi\u00e1c t\u00f9 kh\u00f4ng?<\/strong><\/h3>\n<p style=\"text-align: justify;\">C\u00f3. \u0110\u00e2y l\u00e0 \u0111i\u1ec3m m\u1ea1nh c\u1ee7a \u0111\u1ecbnh l\u00fd cosin so v\u1edbi Pythagore \u2014 \u00e1p d\u1ee5ng \u0111\u01b0\u1ee3c cho tam gi\u00e1c nh\u1ecdn, vu\u00f4ng v\u00e0 t\u00f9. V\u1edbi tam gi\u00e1c t\u00f9, cosin c\u1ee7a g\u00f3c t\u00f9 mang gi\u00e1 tr\u1ecb \u00e2m n\u00ean k\u1ebft qu\u1ea3 t\u00ednh to\u00e1n v\u1eabn ch\u00ednh x\u00e1c.<\/p>\n<h3 style=\"text-align: justify;\"><strong>\u0110\u1ecbnh l\u00fd cosin h\u1ecdc \u1edf l\u1edbp m\u1ea5y?<\/strong><\/h3>\n<p style=\"text-align: justify;\">\u0110\u00e2y l\u00e0 n\u1ed9i dung n\u1eb1m trong ch\u01b0\u01a1ng tr\u00ecnh To\u00e1n l\u1edbp 10 (ph\u1ea7n H\u1ec7 th\u1ee9c l\u01b0\u1ee3ng trong tam gi\u00e1c), c\u00f3 m\u1eb7t trong c\u1ea3 ba b\u1ed9 s\u00e1ch gi\u00e1o khoa hi\u1ec7n h\u00e0nh.<\/p>\n<h3 style=\"text-align: justify;\"><strong>Khi n\u00e0o n\u00ean d\u00f9ng \u0111\u1ecbnh l\u00fd cosin thay v\u00ec \u0111\u1ecbnh l\u00fd sin?<\/strong><\/h3>\n<p style=\"text-align: justify;\">\u01afu ti\u00ean \u0111\u1ecbnh l\u00fd cosin khi d\u1eef ki\u1ec7n \u0111\u1ec1 cho l\u00e0 2 c\u1ea1nh + g\u00f3c xen gi\u1eefa ho\u1eb7c \u0111\u1ee7 3 c\u1ea1nh. D\u00f9ng \u0111\u1ecbnh l\u00fd sin khi c\u00f3 \u00edt nh\u1ea5t m\u1ed9t c\u1eb7p g\u00f3c \u2013 c\u1ea1nh \u0111\u1ed1i di\u1ec7n nhau.<\/p>\n<h3 style=\"text-align: justify;\"><strong>C\u00f4ng th\u1ee9c t\u00ednh di\u1ec7n t\u00edch tam gi\u00e1c c\u00f3 li\u00ean quan \u0111\u1ebfn \u0111\u1ecbnh l\u00fd cosin kh\u00f4ng?<\/strong><\/h3>\n<p style=\"text-align: justify;\">C\u00f3. Sau khi d\u00f9ng \u0111\u1ecbnh l\u00fd cosin t\u00ecm \u0111\u01b0\u1ee3c c\u1ea1nh ho\u1eb7c g\u00f3c c\u00f2n thi\u1ebfu, b\u1ea1n c\u00f3 th\u1ec3 t\u00ednh di\u1ec7n t\u00edch b\u1eb1ng c\u00f4ng th\u1ee9c S = \u00bd\u00b7a\u00b7b\u00b7sinC, ho\u1eb7c d\u00f9ng c\u00f4ng th\u1ee9c Heron n\u1ebfu \u0111\u00e3 bi\u1ebft \u0111\u1ee7 ba c\u1ea1nh. Xem \u0111\u1ea7y \u0111\u1ee7 c\u00e1c c\u00e1ch t\u00ednh trong b\u00e0i <a href=\"https:\/\/hoanghamobile.com\/tin-tuc\/dien-tich-tam-giac\/\"><span style=\"font-weight: 400;\">c\u00f4ng th\u1ee9c t\u00ednh di\u1ec7n t\u00edch tam gi\u00e1c<\/span><\/a>.<\/p>\n<h3 style=\"text-align: justify;\"><strong>\u0110\u1ecbnh l\u00fd cosin c\u00f3 m\u1ea5y d\u1ea1ng c\u00f4ng th\u1ee9c?<\/strong><\/h3>\n<p style=\"text-align: justify;\">C\u00f3 3 c\u00f4ng th\u1ee9c t\u01b0\u01a1ng \u1ee9ng v\u1edbi 3 c\u1ea1nh c\u1ee7a tam gi\u00e1c (t\u00ednh a\u00b2, b\u00b2, c\u00b2), v\u00e0 c\u00f3 th\u1ec3 bi\u1ebfn \u0111\u1ed5i th\u00e0nh 3 c\u00f4ng th\u1ee9c h\u1ec7 qu\u1ea3 t\u01b0\u01a1ng \u1ee9ng \u0111\u1ec3 t\u00ednh cosA, cosB, cosC.<\/p>\n<h2 style=\"text-align: justify;\"><b>K\u1ebft lu\u1eadn<\/b><\/h2>\n<p style=\"text-align: justify;\"><b>\u0110\u1ecbnh l\u00fd cosin<\/b><span style=\"font-weight: 400;\"> l\u00e0 c\u00f4ng c\u1ee5 kh\u00f4ng th\u1ec3 thi\u1ebfu khi gi\u1ea3i c\u00e1c b\u00e0i to\u00e1n tam gi\u00e1c kh\u00f4ng vu\u00f4ng, cho ph\u00e9p t\u00ednh c\u1ea1nh khi bi\u1ebft 2 c\u1ea1nh v\u00e0 g\u00f3c xen gi\u1eefa, ho\u1eb7c t\u00ednh g\u00f3c khi bi\u1ebft \u0111\u1ee7 3 c\u1ea1nh. C\u00f4ng th\u1ee9c a\u00b2 = b\u00b2 + c\u00b2 \u2212 2bc\u00b7cosA v\u1eeba l\u00e0 d\u1ea1ng t\u1ed5ng qu\u00e1t h\u00f3a c\u1ee7a \u0111\u1ecbnh l\u00fd Pythagore, v\u1eeba l\u00e0 n\u1ec1n t\u1ea3ng \u0111\u1ec3 k\u1ebft h\u1ee3p v\u1edbi \u0111\u1ecbnh l\u00fd sin nh\u1eb1m gi\u1ea3i tr\u1ecdn v\u1eb9n m\u1ed9t tam gi\u00e1c b\u1ea5t k\u1ef3. N\u1eafm ch\u1eafc c\u00f4ng th\u1ee9c, h\u1ec7 qu\u1ea3 v\u00e0 c\u00e1c d\u1ea1ng b\u00e0i m\u1eabu \u1edf tr\u00ean l\u00e0 b\u1ea1n \u0111\u00e3 c\u00f3 \u0111\u1ee7 n\u1ec1n t\u1ea3ng \u0111\u1ec3 t\u1ef1 tin x\u1eed l\u00fd ph\u1ea7n H\u1ec7 th\u1ee9c l\u01b0\u1ee3ng trong tam gi\u00e1c.<\/span><\/p>\n<p style=\"text-align: justify;\"><b>Xem th\u00eam:<\/b><\/p>\n<ul>\n<li>\n<p class=\"elementor-heading-title elementor-size-default\"><a href=\"https:\/\/hoanghamobile.com\/tin-tuc\/tap-hop-z-khai-niem-dac-diem-va-ung-dung-cua-so-nguyen-trong-toan-hoc\/\">T\u1eadp h\u1ee3p Z: Kh\u00e1i ni\u1ec7m, \u0111\u1eb7c \u0111i\u1ec3m v\u00e0 \u1ee9ng d\u1ee5ng c\u1ee7a s\u1ed1 nguy\u00ean trong to\u00e1n h\u1ecdc<\/a><\/p>\n<\/li>\n<li>\n<p class=\"elementor-heading-title elementor-size-default\"><a href=\"https:\/\/hoanghamobile.com\/tin-tuc\/tai-phan-mem-geogebra\/\">GeoGebra \u2013 Ph\u1ea7n m\u1ec1m v\u1ebd \u0111\u1ed3 th\u1ecb h\u00e0m s\u1ed1, c\u00f4ng th\u1ee9c To\u00e1n h\u1ecdc<\/a><\/p>\n<\/li>\n<\/ul>\n<p><script type=\"application\/ld+json\">\n{\n  \"@context\": \"https:\/\/schema.org\",\n  \"@type\": \"FAQPage\",\n  \"mainEntity\": [\n    {\n      \"@type\": \"Question\",\n      \"name\": \"\u0110\u1ecbnh l\u00fd cosin d\u00f9ng khi n\u00e0o?\",\n      \"acceptedAnswer\": {\n        \"@type\": \"Answer\",\n        \"text\": \"\u0110\u1ecbnh l\u00fd cosin d\u00f9ng khi bi\u1ebft 2 c\u1ea1nh v\u00e0 g\u00f3c xen gi\u1eefa \u0111\u1ec3 t\u00ednh c\u1ea1nh c\u00f2n l\u1ea1i, ho\u1eb7c bi\u1ebft c\u1ea3 3 c\u1ea1nh \u0111\u1ec3 t\u00ednh m\u1ed9t g\u00f3c b\u1ea5t k\u1ef3 c\u1ee7a tam gi\u00e1c.\"\n      }\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"\u0110\u1ecbnh l\u00fd cosin kh\u00e1c \u0111\u1ecbnh l\u00fd Pythagore \u1edf \u0111i\u1ec3m n\u00e0o?\",\n      \"acceptedAnswer\": {\n        \"@type\": \"Answer\",\n        \"text\": \"\u0110\u1ecbnh l\u00fd Pythagore ch\u1ec9 \u00e1p d\u1ee5ng cho tam gi\u00e1c vu\u00f4ng (a\u00b2 = b\u00b2 + c\u00b2), c\u00f2n \u0111\u1ecbnh l\u00fd cosin \u00e1p d\u1ee5ng cho m\u1ecdi tam gi\u00e1c v\u00e0 c\u00f3 th\u00eam s\u1ed1 h\u1ea1ng \u22122bc\u00b7cosA. Khi g\u00f3c A = 90\u00b0, c\u00f4ng th\u1ee9c thu v\u1ec1 \u0111\u00fang \u0111\u1ecbnh l\u00fd Pythagore.\"\n      }\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"L\u00e0m sao nh\u1edb c\u00f4ng th\u1ee9c \u0111\u1ecbnh l\u00fd cosin nhanh?\",\n      \"acceptedAnswer\": {\n        \"@type\": \"Answer\",\n        \"text\": \"B\u1ea1n c\u00f3 th\u1ec3 nh\u1edb theo m\u1eabu: b\u00ecnh ph\u01b0\u01a1ng c\u1ea1nh \u0111\u1ed1i b\u1eb1ng t\u1ed5ng b\u00ecnh ph\u01b0\u01a1ng hai c\u1ea1nh k\u1ec1 tr\u1eeb \u0111i 2 l\u1ea7n t\u00edch hai c\u1ea1nh k\u1ec1 nh\u00e2n cosin g\u00f3c xen gi\u1eefa. 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Sau khi d\u00f9ng \u0111\u1ecbnh l\u00fd cosin t\u00ecm \u0111\u01b0\u1ee3c c\u1ea1nh ho\u1eb7c g\u00f3c c\u00f2n thi\u1ebfu, b\u1ea1n c\u00f3 th\u1ec3 t\u00ednh di\u1ec7n t\u00edch b\u1eb1ng c\u00f4ng th\u1ee9c S = \u00bd\u00b7a\u00b7b\u00b7sinC, ho\u1eb7c d\u00f9ng c\u00f4ng th\u1ee9c Heron n\u1ebfu \u0111\u00e3 bi\u1ebft \u0111\u1ee7 ba c\u1ea1nh.\"\n      }\n    },\n    {\n      \"@type\": \"Question\",\n      \"name\": \"\u0110\u1ecbnh l\u00fd cosin c\u00f3 m\u1ea5y d\u1ea1ng c\u00f4ng th\u1ee9c?\",\n      \"acceptedAnswer\": {\n        \"@type\": \"Answer\",\n        \"text\": \"C\u00f3 3 c\u00f4ng th\u1ee9c t\u01b0\u01a1ng \u1ee9ng v\u1edbi 3 c\u1ea1nh c\u1ee7a tam gi\u00e1c (t\u00ednh a\u00b2, b\u00b2, c\u00b2), v\u00e0 c\u00f3 th\u1ec3 bi\u1ebfn \u0111\u1ed5i th\u00e0nh 3 c\u00f4ng th\u1ee9c h\u1ec7 qu\u1ea3 t\u01b0\u01a1ng \u1ee9ng \u0111\u1ec3 t\u00ednh cosA, cosB, cosC.\"\n      }\n    }\n  ]\n}\n<\/script><\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u0110\u1ecbnh l\u00fd cosin l\u00e0 c\u00f4ng th\u1ee9c li\u00ean h\u1ec7 gi\u1eefa ba c\u1ea1nh v\u00e0 m\u1ed9t g\u00f3c c\u1ee7a tam gi\u00e1c: b\u00ecnh ph\u01b0\u01a1ng m\u1ed9t c\u1ea1nh b\u1eb1ng t\u1ed5ng b\u00ecnh ph\u01b0\u01a1ng hai c\u1ea1nh c\u00f2n l\u1ea1i tr\u1eeb \u0111i hai l\u1ea7n t\u00edch hai c\u1ea1nh \u0111\u00f3 nh\u00e2n cosin g\u00f3c xen gi\u1eefa. C\u00f4ng th\u1ee9c t\u1ed5ng qu\u00e1t: a\u00b2 = b\u00b2 + c\u00b2 \u2212 2bc\u00b7cosA. \u0110\u00e2y l\u00e0 [&hellip;]<\/p>\n","protected":false},"author":84,"featured_media":565249,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_yoast_wpseo_focuskw":"\u0111\u1ecbnh l\u00fd cosin","_yoast_wpseo_title":"\u0110\u1ecbnh L\u00fd Cosin: C\u00f4ng Th\u1ee9c, Ch\u1ee9ng Minh & B\u00e0i T\u1eadp L\u1edbp 10","_yoast_wpseo_metadesc":"\u0110\u1ecbnh l\u00fd cosin l\u00e0 g\u00ec? C\u00f4ng th\u1ee9c a\u00b2=b\u00b2+c\u00b2\u22122bc\u00b7cosA, h\u1ec7 qu\u1ea3 t\u00ednh g\u00f3c, c\u00e1ch ch\u1ee9ng minh v\u00e0 b\u00e0i t\u1eadp c\u00f3 l\u1eddi gi\u1ea3i chi ti\u1ebft cho h\u1ecdc sinh l\u1edbp 10.","footnotes":""},"categories":[29235,3910,30453],"tags":[],"class_list":["post-565248","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-goc-to-mo","category-kham-pha","category-kien-thuc"],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.3 (Yoast SEO v27.4) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>\u0110\u1ecbnh L\u00fd Cosin: C\u00f4ng Th\u1ee9c, Ch\u1ee9ng Minh &amp; B\u00e0i T\u1eadp L\u1edbp 10<\/title>\n<meta name=\"description\" content=\"\u0110\u1ecbnh l\u00fd cosin l\u00e0 g\u00ec? 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