a2=a1+2a3=a2+2^2=a1+2+2^2a4=a3+2^3=a1+2+2^2+2^3.an=an-1+2^(n-1)=a1+2+2^2+.+2^(n-1) （这里a1=5,后面是首项为2,公比为2,项数为n-1项等比数列的和）=5+2[1-2^(n-1)]/(1-2)=5+2^n-2=3+2^n