因为a(n+1)=2an+3 所以a(n+1)+3=2(an+3)
[a(n+1)+3]/(an+3)=2
{an+3}是以a1+3=4位首项,2为公比的等比数列
an+3=4*2^(n-1)
an=4*2^(n-1)-3