因为(an,an+1)在函数f(x)=x^2+2x的图象上 an+1=an^2+2an 1+an+1=an^2+2an +1=(1+an)^2 lg(1+an+1)=2lg(1+an),（n>=1）,bn=lg(an+1),则b(n+1) =2 bn,{bn}是等比数列,b1= lg(1+a1)=lg3.bn= b1*2^(n-1),bn= lg3*2^(n-1)...