cos(y+π/2)=-siny吗?如果相等那么是根据诱导公式吗?

cos(y+π/2)=-siny吗?如果相等那么是根据诱导公式吗? 证明cosx-cos(x+2y)/2siny= sin(x+y)证明cosX-cos(x+2y)/2siny= sin(x+y) 证明tan(90°-y)=(2cos^3y-cosy)/(siny-2sin^3y) 证明sinx+cos(x+y)siny/cosx-sin(x+y)siny=tan(x+y) 请问,如何证明sinx+siny=2*sin(x+y/2)*cos(x-y/2) 为什么sin（x+y) - sinx = 2cos(x+y/2)siny/2 求推导sinx-siny=2cos（(x+y)/2)sin((x-y)/2) 求证sinx+siny=2sin(x+y)/2*cos(x-y)/2 求证:sinx+siny=2sin(x+y)/2cos(x-y)/2 求证|sinx-siny|=|2sin[(x-y)/2]cos[(x+y)/2]| sin(x+y)-sinx=2siny/2·cos(x+y/2)怎么来的 证明 [sin(2x+y)/sinx]-2cos(x+y)=siny/sinx 求证sin（2x+y)/sinx-2cos(x+y)=siny/sinx 求证：sin(2x+y)/sinx-2cos(x+y)=siny/sinx sinx+siny=2/5,cosx+cosy=6/5,求cos(2x-2y) 若cosx*cosy+sinx*siny=1/3 ,则cos(2x-2y)=? cosx+cosy=1/2,sinx-siny=1/3,则cos(x+y)=? 已知cosx+cosy=1/2,sinx-siny=1/3,则cos(x+y)=