009A Sample Final 1, Problem 6

Consider the following function:

$f(x)=3x-2\sin x+7$

a) Use the Intermediate Value Theorem to show that $f(x)$ has at least one zero.

b) Use the Mean Value Theorem to show that $f(x)$ has at most one zero.

Foundations:

Solution:

(a)

Step 1:
First note that $f(0)=7$.
Also, $f(-5)=-15-2\sin(-5)+7=-8-2\sin(-5)$.
Since $-1\leq \sin(x)\leq 1$,
$-2\leq -2\sin(x)\leq 2$.
Thus, $-10\leq f(-5)\leq -6$ and hence $f(-5)<0$.

Step 2:
Since $f(-5)<0$ and $f(0)>0$, there exists $x$ with $-5<x<0$ such that
$f(x)=0$ by the Intermediate Value Theorem. Hence, $f(x)$ has at least one zero.

(b)

Step 1:
We have $f'(x)=3-2\cos(x)$. Since $-1\leq \cos(x)\leq 1$,
$-2\leq -2\cos(x)\leq 2$. So, $1\leq f'(x)\leq 5$.
Therefore, $f'(x)$ is always positive.

Step 2:
Since $f'(x)$ is always positive, $f(x)$ is an increasing function.
Thus, $f(x)$ has at most one zero.

Final Answer:
(a) Since $f(-5)<0$ and $f(0)>0$, there exists $x$ with $-5<x<0$ such that
$f(x)=0$ by the Intermediate Value Theorem. Hence, $f(x)$ has at least one zero.
(b) Since $f'(x)$ is always positive, $f(x)$ is an increasing function.
Thus, $f(x)$ has at most one zero.

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