f（x）=cos²x+sinx+a-1
=-sin^2 x+sinx+a
u=sinx,-1≤u≤1
f（x）=-(u-1/2)^2+a+1/4,
a-2≤f（x）≤a+1/4,
a-2=1,且a+1/4=17/4,
a=3, 且a=4.
无解.