Wednesday, 5 February 2014

calculus - Limit of $frac{tan(x)-x}{x^3}$ as $x$ approaches $0$ without L'Hospital's Rule




I am trying to find the limit of $\frac{\tan(x)-x}{x^3}$ as $x$ approaches $0$. I know that this can be found by using L'Hospital's Rule 3 times. Is there a way to solve this problem without using L'Hospital's Rule?



Please do not use Taylor series; I consider this to be an equivalent method. I have noticed that the required number of applications of L'Hospital's Rule is precisely the order of the first non-zero derivative, which I think is essentially because a product is $0$ if and only if at least one factor is $0$.


Answer



you can simplify $$\frac{\tan x - x}{x^3}= \frac{\sin x - x \cos x}{x^3\cos x } = \frac{x - \frac{x^3}{6}+\cdots - x\left(1 - \frac{x^2}2+\cdots\right)}{x^3} = \frac 13 \text{ as } x \to 0. $$


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