已知cos(x-y/2)=-1/9,sin(x/2-y)=2/3,0
问题描述：
已知cos(x-y/2)=-1/9,sin(x/2-y)=2/3,0
答
0
sin(x-y/2)=√(1-cos(x-y/2)^2)=4√5/9,cos(x/2-y)=√(1-sin(x/2-y)^2=√5/3.
cos(x+y)=2[cos(x/2+y/2)]^2-1=2{cos[x-y/2-(x/2-y)]}^2-1=2[cos(x-y/2)cos(x/2-y)+sin(x-y/2)sin(x/2-y)]^2-1=2(7√5/27)^2-1=490/729-1=-239/729
答
y=cosx/(2cosx+1)
=(2cosx+1)y=cosx
=2ycosx+y=cosx
y=(1-2y)cosx
y=1/2时
0=1/2
y不等于1/2