sina+sinb=a,cosa+cosb=b,ab不等于0,cos（a-b）等于

求证：cos(a+b)=cosa*cosb-sina*sinb 证明:cos(a+b)=cosa×cosb-sina×sinb 已知cos(a-b)=3/1,求(sina+sinb)(sina+sinb)+(cosa+cosb)(cosa+cosb)= 若sina+sinb=1,cosa+cosb=o求cos^2a+cos^2b sina+sinb=a,cosa+cosb=b,ab不等于0,cos（a-b）等于 sin(a+b)cos(a-b)=sina*cosa+sinb*cosb 求证:sin(a+b)cos(a-b)=sina*cosa+sinb*cosb麻烦个位 帮个忙~ sinA(cos(2A+B)+cosB)=cosA(sin(2A+B)-sinB)证明 证明下列恒等式：sin(a+b)*cos(a-b）=sina*cosa+sinb*cosb cos(A+B)+sin(A-B)=(cosA+sinA)(cosB-sinB) cos(A-B)-sin(A+B)=(cosA-sinA)(cosB-sinB). 已知sina+sinb+siny=0,cosa+cosb+cosy=0,则cos（a-b)=? 已知sina+sinB=1/2,cosa+cosB=1/3,则cos(a-B)= sina+sinb=1/4 cosa+cosb=1/3 则cos(a-b)= 若sina+sinb+sinr=0,cosa+cosb+cosr=0,则cos(a-b)=? sina+sinb=1,cosa+cosb=0,求cos(a+b)=? 已知sina+sinb=siny,cosa+cosb=cosy,求证cos(a-y)=1/2 已知cosa+cosb=1/2,sina+sinb=1/3,求cos(a-b)rt.