f(x)=2cos^2+3sinx
=2(1-sin^2x)+3sinx
=-2(sinx-3/4)^2+25/8
x属于[-π/2,π/2]
sinx的取值为[-1,1]
故sinx=3/4时,最大值为25/8
sinx=-1时,最小值为-3