(sinx)^2-2cosx-2a=0
2a=1-(cosx)^2-2cosx
a=-1/2(cosx)^2-cosx+1/2
cosx=t
-1≤t≤1
a=-1/2t^2-t+1/2=-1/2(t+1)^2+1
t=-1时
a=1
t=1时
a=-1/2
a的范围[-1/2,1]