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# Prove that the recursive sequence xn+1 = (1+xn)1/2 converges

Prove that the sequence whose terms are defined recursively by

converges, and compute the limit of the sequence.

Proof. To show the sequence converges we show that it is monotonically increasing and bounded above. To see that it is monotonically increasing we use induction to prove that

For the case we have

Since , the statement holds in the case . Assume then that the statement holds for some positive integer . Then,

since by the induction hypothesis. Hence, so by induction for all positive integers . Hence, the sequence is monotonically increasing.
Next we use induction again to prove the sequence is bounded above by . For we have so the hypothesis holds. Assume then that for all positive integers up to . Then,

Hence, for all positive integers .
This shows that the sequence converges

To compute the limit, assume the sequence converges to a number (we just proved that it converges, so this assumption is valid). Then we have

(We can discard the negative solution since to the quadratic at the end since the sequence is certainly all positive terms.)

1. C. Taylor says:

goated example/explanation, kisses!!!

2. Michelle says:

Thank you very much for this explanation. It makes perfect sense and was a great help

3. Anonymous says:

For the first inductive step, how do we know that xk+2 – xk+1 is greater than 0?

• John Kameas says:

They arrived at the result because they used the assumption x_(k+1)>x_k .