qn 3天以前 cos∠BAC = (5²+7²-6²)/(2·5·7) = 19/35 cos∠EAG = cos(180°-∠BAC) = -cos∠BAC = - 19/35 EG² = 5² + 7² - 2·5·7·(-19/35) = 25 + 49 + 38 = 112 EG = √112 = 4√7 留言