求值域y=-2cos^2x+2sinx+3/2，x∈[-π/4，π/4]

y=-2cos^2x+2sinx+3/2
=-2（1-sin^2x）+2sinx+3/2
=-2sin^2x+2sin^2x-1/2
=-2（sinx-1/2）^2
-π/4