cos^2A - cos^2B + sin^2C=2cosA *sinB *sinC证明

cos^2A - cos^2B + sin^2C=2cosA *sinB *sinC证明 cos^2A - cos^2B + sin^2C=2cosA *sinB *sinC sina+sinb+sinc=0 cosa+cosb+cosc=0求证cos*2a+cos*2b+cos*2c=3|2 在△ABC中,求证:sinA+sinB+sinC=4cos(A/2)cos(B/2)cos(C/2) 11.在△ABC中,求证sinA+sinB+sinC=4cos(A/2)cos(B/2)cos(C/2) sinb/sina=cos(a+b),证明3sinb=sin(2a+b) 求证： sina+sinb=2sin[(a+b)/2]cos[(a-b)/2] cosa+cosb=2cos[(a+b) 不等式题目：在△ABC中请证明sinA+sinB+sinC≤cos(A/2)+cos(B/2)+cos(C/2) a,b,c属于(0,π/2),a=cosa,b=sin(cosb) c=cos(sinc) 试比较a,b,c大小 要详细 如何证明sinA+sinB=2sin((A+B)/2)cos((A-B)/2) 证明sin(2a+b)/sina-2cos(a+b)=sinb/sina 在三角形ABC中求证sinA+sinB+sinC=4cos（A/2)COS(B/2)COS(C/2)证到这步然后怎么证: