a(n+1)/an=2[(n+1)/n],所以,
a2/a1=2[2/1],a3/a2=2[3/2],a4/a3=2[4/3],…,an/a(n-1)=2[n/(n-1)],累乘,得:
an/a1=2^(n-1)×[n],所以,an=n×2^n.