Question

三角函数的解三角形的公式已知△ABC,求证sin(B+C)/2 = cosA/2

Answer 1

已知△ABC

∴A+B+C=180°

∴B+C=180°-A

∴sin[(B+C)/2] = sin[(180°-A)/2] = sin(90°-A/2) = cos(A/2)