f(x)=cos2x-sinx+1 =2-2sin^2x-sinx
=-2(sinx+1/4)^2+2+1/8=-2(sinx+1/4)^2+17/8
因为x∈[3/4π,3/2π],所以：sinx∈[根号2/2,-1]
故f(x)的最大值是：17/8,此时x=5π/4;
当x=3π/2时,最小值是：1