已知sin A+sinB +siny=0 ,cos A+cos B+cosy=0,求cos(B -y)的值.

已知sin A+sinB +siny=0 ,cos A+cos B+cosy=0,求cos(B -y)的值. 已知sina+sinb+siny=0,cosa+cosb+cosy=0,则cos（a-b)=? 已知sina+sinb+siny=0,cosa+cosb+cosy=0.(1)求cos(a-b)的值;(2)若a.b.y属于【0,3分之4派】求sin(a+b+y)值 已知锐角abc满足sina-sinb+siny=0,cosa-cosb-cosy=0,求a-b 已知sina+sinb=siny,cosa+cosb=cosy,求证cos(a-y)=1/2 三角函数不等式1)已知x+y+z=∏/2,x,y,z>0,求证:8(cosx·cosy·cosz)²≥27sinx·siny·sinz2)已知A+B+C=∏,A,B,C>0,试确定1/sin(A/2)+1/sin(B/2)+1/sin(C/2)与(1/sinA+1/sinB+1/sinC)(sinA+sinB+sinC)·2/3的大小关系,并证明.1)已知x 若sina+sinb+siny=cosa+cosb+cosy=0,则cos(a-b)等于 已知cosa=cosx*siny,cosb=sinx*siny求证：sin^2a+sin^2b+sin^2y=2 已知sina-cosb+siny=0,cosa+cosb-cosy=0,求sin（a-b）的值 已知锐角a b y 且 sina+siny=sinb,cosa-cosy=cosb,求a-b的值 已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos(b-y)的值 .已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos（b-y）的值.（解题 已知COSa+COSb+COSy=0,SINa+SINb+SINy=0,求COS(b-y)的值 已知cos(a+B)+1=0,求证sin(2a+B)+sinB=0?(提示:sin(-a)=-sina). sina+sinb+siny=0.cosa+cosb-cosy=0,则cos(a-b)的值是, 已知sinα +sinβ+ siny=0,cosα +cosβ +cosy=0,求cos(α-β) 三角函数题目：已知：cosa=cosx*siny cosb=sinx*siny 求证：sin^2*a+sin^2*b+sin^2*ysin^2*a+sin^2*b+sin^2*y 应该是 sin^2a+sin^2b+sin^2y =2 已知A、B为锐角,且sinA*sinA+sinB*sinB=sin(A+B),求证A+B=∏/2