(an,an+1)在函数f(x)=x^2+4x+2的图像上a(n+1)+2=(an+2)^2设cn=an+2 c1=a1+2=3cn=c(n-1)^2=c1^2^(n-1)=3^2^(n-1)an=cn-2=3^2^(n-1)-21/(an+1)=1/(3^2^(n-1)-1) 1/(an+3)=1/(3^2^(n-1)+1)bn是1/an+1与1/an+3的等差中项...