(1)t=a^x是增函数,在[-1,1]上值域是[1/a,a]
f(x)=t^2+2t-1在[1/a,a]上是增函数,最大值为a^2+2a-1=14
解得a=3
(2)t^2+2t-1=7
t=2
x=log3 2