Y=sin^2x-sinx,则函数y的值域?

Y=sin^2x-sinx,则函数y的值域?

Y=sin^2x-sinx
= (sinx-1/2)^2-1/4
-1≤sinx≤1
-3/2≤sinx-1/2≤1/2
0≤(sinx-1/2)^2≤9/4
-1/4≤(sinx-1/2)^2-1/4≤2
值域【-1/4,2】