a(n+1)=an^2+2an=(an+1)^2-1,即a(n+1)+1=(an+1)^2
设an+1=bn
则b(n)=b(n-1)^2=b(n-2)^(2*2).=b(n-(n-1))^(2(n-1))=b1^(2(n-1))
bn=3^(2(n-1))
n≥2时,an=3^(2(n-1))-1
a(n+1)=an^2+2an=(an+1)^2-1,即a(n+1)+1=(an+1)^2
设an+1=bn
则b(n)=b(n-1)^2=b(n-2)^(2*2).=b(n-(n-1))^(2(n-1))=b1^(2(n-1))
bn=3^(2(n-1))
n≥2时,an=3^(2(n-1))-1