a) Ta có: A+B+C = 180
=> Cos(A+C) = -cos(180 - A - C) = -cos B
b) sin( (A+C)/2) = sin((180 - B) /2) = sin(90 - B/2) = cos(B/2)
c) sin(A+B+2C) = sin(180 + C) = -sin C
d) tan((B+C-2A)/2) = tan ((180 - 3A)/2) = tan(90- (3A)/2) =cot (3A)/2