sina+sinb=0,cosa+cosb=2/3,cos2a+cos2b=?sina=-sinb即a=-b，为什么

求证：cos(a+b)=cosa*cosb-sina*sinb 证明:cos(a+b)=cosa×cosb-sina×sinb 已知sina+sinb+siny=0,cosa+cosb+cosy=0,则cos（a-b)=? 若sina+sinb+sinr=0,cosa+cosb+cosr=0,则cos(a-b)=? sinA+sinB+sinY=0 ,cosA+cosB+cosY=0,求cos(B-Y)的值. sinA+sinB+sinC=0; cosA+cosB+cosC=0,求cos(B-C)的值? sina+sinb+sinc=0,cosa+cosb+cosc=0,求cos(B-C)的值? sinA+sinB+sinc=0 cosA+cosB+cosC=0 cos(B-C) sina+sinb=1,cosa+cosb=0,求cos(a+b)=? 若sina+sinb+siny=cosa+cosb+cosy=0,则cos(a-b)等于 已知sina+sinb=1,cosa+cosb=0求cos（a+b)的值 已知sina+sinb=1,cosa+cosb=0,则cos(a-b)的值为? sina+sinb=1,cosa-cosb=0,求cos(a+b) sina+sinb=a,cosa+cosb=b,ab不等于0,cos（a-b）等于 已知cos(a-b)=3/1,求(sina+sinb)(sina+sinb)+(cosa+cosb)(cosa+cosb)= cosa+cosb=0,sina+sinb=1 则cosa=cos(π-b)的证明过程 sinA/sinB=cosB/cosA 已知（1+cosa-sinb+sina*sinb=0,1-cosa-cosb+sina*cosb=0),则sina的值为