a(n+1)
=f(2^n+1)
=f(2*2^n)
=2f(2^n)+2^nf(2)
=2an+2^(n+1)
故:a(n+1)/2^(n+1)-an/2a^n=1,
即an/2^(n)=n
所以an=n2^n,