令bn=an+3
b(n+1)
=a(n+1)+3
=2an+6
=2(an+3)
=2bn
可以看到此时数列bn的公比为2
且b1=a1+3=8=2³
bn=2³×2^(n-1)=2^(n+2)
因此an=bn-3=2^(n+2)-3