f（x）=cos^2x--4sinx
=1-sin^2x-4sinx
设sinx=t
则-1<=t<=1
原式=-t^2-4t+1=5-(t+2)^2
即值域为[-4,4]能把1-sin^2x怎么来的解释下么谢谢我懂了之后给加20分sin^2x+cos^2x=1所以cos^2x=1-sin^2x