Tuesday, 7 April 2015

calculus - Proof of limlimitsxtoinftyxsinx divergences to not infinity(infty,infty)

lim divergences to not infinity(\infty, -\infty)




1. Prove \lim\limits_{x \to \infty}x\sin x \neq \infty, -\infty
For \lim\limits_{x \to \infty}x\sin x = \infty, N must exist in below:
For all M>0, there exists N such that
x>N \implies x\sin x > M
But xsin x has lower bound and upper bound for every x (-x\le x\sin x\le x (x>0))
So x>N \implies x\sin x > M can't be satisfied for every M and N does not exist
Therefore \lim\limits_{x \to \infty}x\sin x \neq \infty
We can prove \lim\limits_{x \to \infty}x\sin x \neq -\infty in similar way





2. Prove \lim\limits_{x \to \infty}x\sin x \neq L for any constant L
For \lim\limits_{x \to \infty}x\sin x = L, N and L must exist in below:
For all \epsilon>0, there exists N and L such that
x>N \implies |x\sin x-L| < \epsilon
|xsin x-L| has following possibilities
1) |-x-L| \le |x\sin x-L| \le |x-L|
2) |x-L| \le |x\sin x-L| \le |-x-L|
3) |x\sin x-L| \le |-x-L|, |x\sin x-L| \le |x-L|
So |x\sin x-L| < \epsilon can't be satisfied for every \epsilon,L and N,L does not exists
Therefore \lim\limits_{x \to \infty}x\sin x \neq L for every L



Is this proof correct?

No comments:

Post a Comment

real analysis - How to find lim_{hrightarrow 0}frac{sin(ha)}{h}

How to find \lim_{h\rightarrow 0}\frac{\sin(ha)}{h} without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...