Thursday, May 3, 2018

Single Variable Calculus, Chapter 4, 4.2, Section 4.2, Problem 16

Show that there is no value of $c$ such that $f(3) -f(0) = f'(c)(3-0)$ in the equation $f(x) = 2 - |2x - 1|$. Why does this not contradict the Mean Value Theorem?
Using the definition of absolute value, we can rewrite $f(x)$ as

$
f(x) =
\begin{array}{c}
2 - (2x - 1) & \text{ for } & x \geq \frac{1}{2}\\
2 - (-(2x-1)) & \text{ for } & x < \frac{1}{2}
\end{array}
\qquad \Longrightarrow \qquad f(x)=
$
$
\begin{array}{c}
-2x + 3 & \text{ for } & x \geq \frac{1}{2}\\
2x + 1 & \text{ for } & x < \frac{1}{2}
\end{array}
$


If we evaluate $f(x)$ for $\displaystyle x \geq \frac{1}{2}$. By using Mean Value Theorem, we get $\displaystyle f'(c) = \frac{f(b) - f(a)}{b-a}$

$
\begin{equation}
\begin{aligned}
f'(c) &= \frac{\left[ -2(3) + 3\right] - \left[ -2(0) + 3 \right]}{3-0}\\
\\
f'(c) &= -2 \text{ for } c \geq \frac{1}{2}\\
\\
\text{for } x < \frac{1}{2},\\
\\
f'(c) &= \frac{\left[ -2(3) + 1\right] - \left[ -2(0) + 1 \right]}{3-0}\\
\\
f'(c) &= 2 \text{ for } c < \frac{1}{2}\\
\end{aligned}
\end{equation}
$

We got two different values of $f'(c)$ and it shows that $f(x)$ is not differentiable at $x = \frac{1}{2}$ the function is continuous on the interval $[0,3]$ (See the graph below).



Therefore, it does not satisfy the Mean Value Theorem.

No comments:

Post a Comment

Why is the fact that the Americans are helping the Russians important?

In the late author Tom Clancy’s first novel, The Hunt for Red October, the assistance rendered to the Russians by the United States is impor...