.已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos（b-y）的值.（解题

.已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos（b-y）的值.（解题 已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos(b-y)的值 已知COSa+COSb+COSy=0,SINa+SINb+SINy=0,求COS(b-y)的值 已知sina+sinb+siny=0,cosa+cosb+cosy=0.(1)求cos(a-b)的值;(2)若a.b.y 三角恒等变换1已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos(b-y)的值 已知sina+sinB+siny=o,cosa+cosB+cosy=o,哪么cos(B-y)的值为多少 已知锐角a b y 且 sina+siny=sinb,cosa-cosy=cosb,求a-b的值 sina+sinb+siny=0.cosa+cosb-cosy=0,则cos(a-b)的值是, 若sina+sinb+siny=cosa+cosb+cosy=0,则cos(a-b)等于 已知sina+sinb=siny,cosa+cosb=cosy,求证cos(a-y)=1/2 已知锐角abc满足sina-sinb+siny=0,cosa-cosb-cosy=0,求a-b 已知sina+sinβ+siny=0,cosa+cosβ+cosy=0,求cos(β-y)的值?