y=cos2x+3sinx
=1-2sin^x+3sinx
=1-2(sin^x-3/2sinx+9/16)+9/8
=-2(sinx-3/4)^+17/8
-2(sinx-3/4)^的最大值为0,最小值为sinx=-1时,=-49/8
可得最大值为17/8,最小值-4