F(a+2)=3^(a+2)=18
9*3^a=18
3^a=2
a=log3 2(3为底,2的对数)
g(x)=λ*3^(ax-4x)
g'(x)=λ(a-4)ln3*3^(ax-4x)
当0≤x≤1时,因a=log3 2