Do you know exponential

Calculus Level 4

True or false

Let P ( x ) = 1 + x + x 2 2 ! + x 3 3 ! + + x n n ! P(x) = 1 + x + \dfrac{x^2}{2!} + \dfrac{x^3}{3!} + \cdots + \dfrac{x^n}{n!} , where n n is a very large positive integer

Then lim x e x P ( x ) = 1 \displaystyle\lim_{x\rightarrow \infty} \dfrac{e^x}{P(x)} = 1

False True

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2 solutions

Tapas Mazumdar
Jun 20, 2017

lim x e x P ( x ) = lim x r = 0 x r r ! r = 0 n x r r ! = lim x r = 0 n x r r ! + r = n + 1 x r r ! r = 0 n x r r ! = lim x [ 1 + r = n + 1 x r r ! r = 0 n x r r ! as x ] \begin{aligned} \lim_{x \to \infty} \dfrac{e^x}{P(x)} &= \lim_{x \to \infty} \dfrac{\displaystyle \sum_{r=0}^{\infty} \frac{x^r}{r!}}{\displaystyle \sum_{r=0}^{n} \frac{x^r}{r!}} \\ &= \lim_{x \to \infty} \dfrac{\displaystyle \sum_{r=0}^{n} \frac{x^r}{r!} + \sum_{r=n+1}^{\infty} \frac{x^r}{r!}}{\displaystyle \sum_{r=0}^{n} \frac{x^r}{r!}} \\ &= \lim_{x \to \infty} \left[ 1 + \underbrace{\dfrac{\displaystyle \sum_{r=n+1}^{\infty} \frac{x^r}{r!}}{\displaystyle \sum_{r=0}^{n} \frac{x^r}{r!}}}_{\to \infty \text{ as } x \to \infty} \right] \end{aligned}

Thus limit diverges and statement is false .

Akeel Howell
Jun 19, 2017

The problem states that n n is a large positive integer, however infinity is not a number and so the limit would only equal 1 1 if P ( x ) P(x) was an infinite series instead of a partial sum terminating at some large positive integer n n . The limit in question is undefined.

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