点(an,a(n+1))在函数f(x)=x^2+2x
a(n+1)=an^2+2an
a(n+1)+1=an^2+2an+1=(an+1)^2
令bn=1+an
b1=1+a1=3
则bn=b(n-1)^2=...=b1^[2(n-1)]=3^[2(n-1)]
lg(1+an)=lgbn=2(n-1)lg3