Question

A、B、C是三角形ABC的三个内角,则sinA+B/2等于

Answer 1

cosC/2。理由如下：因为sinA=cos(90-A),sinB=cos(90-B),所以sinA+B/2=cosC/2

Answer 2

反推
sin[(a+b)/2]=cosc/2
sin[(a+b)/2]=sin(90-c/2)
sin[(a+b)/2]=sin[(180-c)/2]
a+b=180-c
a+b+c=180