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问题: 又一个极限

1.设a1=2,an+1=1/2(an+1/an)(n=1,2,...),
试证明:lim an=1

解答:

A(n+1) =(1/2)(An +1/An) > 0
==> A(n+1) =(1/2)(An +1/An) >= (1/2)*2 =1 ...(1)
A1 =2, A(n+1)-An = (1/2)(1 -An^2)/An < 0 ...(2)
(1)(2) ==> lim an=1