求函数y=cos2x+asinx+2的最大值g(a),并求出g(a)=5时,

(1):y = cos2x + asinx +2
=(1-2sin^2x)+asinx +2
=-2(sin^2)x + asinx +3
令 t = sinx 则：-1 小于等于 t 小于等于 1
故：原式 = -2t^2 + at +3
当 t = (a)/(-2*(-2))=a/4 的时候上式取的最大值g(a)
g(a) = -2*(a/4)^2 + a*(a/4) +3
= (a^2)/8 +3;
(2):g(a) = 5
则：(a^2)/8 +3 = 5
得：a = |4|