已知得 X属于R
原等式=1-cos^2(x)-2cosx
=-(cos^2(x)+2cos(x))+1
=-(cos(x)+1)^2 +1 +1
因为 0 =< cos（x）+1 < =2
所以 -4 <= -(cos(x)+1)^2 <= 0
所以 -2 <= -(cos(x)+1)^2 +1 +1 <= 2
所以值域是 [-2,2]