Show that there is no positive integer n for which √n-1 + √n+ 1 is rat...
Proof that there is no positive integer n for which √n-1 √n 1 is rational
Assumption
We assume that there exists a positive integer n such that √n-1 √n 1 is rational.
Deriving a Contradiction
Let's simplify the expression by multiplying ( √n-1 + 1) with (√n-1 - 1):
(√n-1 + 1) (√n-1 - 1) = (√n-1)² - 1
= n - 2√n + 1 - 1
= n - 2√n
We know that √n is irrational if n is not a perfect square. So, if √n is irrational, then n - 2√n is also irrational.
If √n is rational, then we can express it as a fraction p/q, where p and q are coprime integers. Substituting this value in n - 2√n, we get:
n - 2√n = n - 2(p/q) = (nq - 2p)/q
Since p and q are coprime, nq - 2p and q are also coprime. Hence, n - 2√n is irrational.
Thus, we have derived a contradiction. Hence, our assumption that there exists a positive integer n such that √n-1 √n 1 is rational is false.
Conclusion
Therefore, there is no positive integer n for which √n-1 √n 1 is rational.