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,则cos（a-b)=? 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 已知锐角abc满足sina-sinb+siny=0,cosa-cosb-cosy=0,求a-b 已知COSa+COSb+COSy=0,SINa+SINb+SINy=0,求COS(b-y)的值 已知sina+sinb=siny,cosa+cosb=cosy,求证cos(a-y)=1/2 sinA+sinB+sinY=0 ,cosA+cosB+cosY=0,求cos(B-Y)的值. 若sina+sinb+siny=cosa+cosb+cosy=0,则cos(a-b)等于 已知锐角a b y 且 sina+siny=sinb,cosa-cosy=cosb,求a-b的值 已知sina+sinB+siny=o,cosa+cosB+cosy=o,哪么cos(B-y)的值为多少 已知sina=sinb=siny=0,cosa=cosb=cosy=0,求cos(b-y)的值 三角恒等变换1已知sina+sinb+siny=0,cosa+cosb+cosy=0,求cos(b-y)的值 sina+sinb+siny=0.cosa+cosb-cosy=0,则cos(a-b)的值是, (sina/sinb)^2+(cosacosy)^2=1求证(tana/tanb)^2=(siny)^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 在△ABC中,求证：a(sinB-sinC)+b(sinC-sinA)+c(sinA-sinB)=0 已知a/sinA=b/sinB,如何求b/sinB=c/sinC 在三角形ABC中,已知（sinA+sinB+sinC）乘以（sinA+sinB-sinC）=3sinA乘以sinB.求C的大小要详细过程