cos(x-y/2)=-1/9,sin(x/2-y)=2/3,π/2

Sin x-sin y=2/3 cos x-cos y=1/2 求cos(x-y) y =(cos^2) x - sin (3^x),求y' sin(x+y)sin(x-y)=k,求cos^2x-cos^2y 化简y=sin^2(x)+2sin(x)cos(x)+3cos^2(x) 证明COS（X+Y）COS（X-Y）=COS^2X-SIN^2Y 已知cos(x-y/2)=-1/9,sin(x/2-y)=2/3,0 求导y=(1+sin^2 x)/(cos(x^2)) sin^2x+cos^2y=1/2 求3sin^2x+sin^2y的最值 求证sin^2x+sin^2y-sin^2x*sin^2y+cos^2x*cos^2y=1 设sin(x+y)sin(x-y)=m,则cos^2x-cos^2y的值 y=sin^2x-3sinxcosx+4cos^2x是否等于y=sin^2x+cos^2x+3cos^2x-3/2*si y=sin^2x+sinxcoswx+cos^2x 化简