Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 3: Derivatives - Section 3.1 - Tangents and the Derivative at a Point - Exercises 3.1 - Page 109: 35

Answer

The graph of $f(x)$ has a tangent at the origin.

Work Step by Step

$f'(0)=\lim_{h\to0}\frac{f(h+0)-f(0)}{h}=\lim_{h\to0}\frac{f(h)-f(0)}{h}$ As we know that $h\to 0$, $h$ does not equal $0$, so $f(h)=h^2\sin(\dfrac{1}{h})$ and for $f(0)$ at $x=0 \implies f(0)=0$ Now, $f'(0)=\lim\limits_{h\to 0}\dfrac{h^2\sin(1/h)-0}{h}=\lim\limits_{h\to 0} \dfrac{h^2\sin(1/h)}{h}=\lim\limits_{h\to 0}(h\sin(1/h)\Big)$ Also, $-1\le\sin(1/h)\le1$ This implies that , $-h\leq h\sin(\dfrac{1}{h})\leq h$ As per the Sandwich Theorem, we have $f'(0)=\lim\limits_{h\to 0}(h\sin(\dfrac{1}{h}))=0$ This means that the slope of the tangent to $f(x)$ is at the origin. and the graph of $f(x)$ has a tangent at the origin.
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