University Calculus: Early Transcendentals (3rd Edition)

Published by Pearson
ISBN 10: 0321999584
ISBN 13: 978-0-32199-958-0

Chapter 2 - Practice Exercises - Page 111: 49

Answer

$$\lim_{x\to\infty}\frac{x+\sin x+2\sqrt x}{x+\sin x}=1$$

Work Step by Step

*Remember that $\lim_{x\to\pm\infty}\frac{a}{x^n}=a\lim_{x\to\pm\infty}\frac{1}{x^n}=a\times0=0$ $$A=\lim_{x\to\infty}\frac{x+\sin x+2\sqrt x}{x+\sin x}$$ Divide both numerator and denominator by the highest degree of $x$ in the denominator, which is $x$ here: $$A=\lim_{x\to\infty}\frac{1+\frac{\sin x}{x}+\frac{2}{\sqrt x}}{1+\frac{\sin x}{x}}$$ $$A=\frac{1+\lim_{x\to\infty}\frac{\sin x}{x}+0}{1+\lim_{x\to\infty}\frac{\sin x}{x}}=\frac{1+\lim_{x\to\infty}\frac{\sin x}{x}}{1+\lim_{x\to\infty}\frac{\sin x}{x}}$$ We know that $-1\le\sin x\le1$, hence $\frac{-1}{x}\le\frac{\sin x}{x}\le\frac{1}{x}$ (because $x\to\infty$, we assume $x\gt0$ here) Furthermore, $\lim_{x\to\infty}-1/x=\lim_{x\to\infty}1/x=0$ Therefore, according to Sandwich Theorem, $\lim_{x\to\infty}(\sin x/x)=0$ $$A=\frac{1+0}{1+0}=1$$
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