I'm getting better at using trig functions, but this problem has me up against a wall.
As I understand it, to find if a function is increasing or decreasing over a given interval, select a number inside that interval and if the result is greater or less than zero, the interval is increasing and decreasing, respectively.
Given $f(x)=5x+10 \sin x$ on the interval $(0, 2\pi)$, find the open intervals where the function is increasing or decreasing.
The given intervals are $(0, \frac{2\pi}{3}), (\frac{2\pi}{3}, \frac{4\pi}{3}), (\frac{4\pi}{3}, 2\pi).$
On the first interval, I selected $\frac{\pi}{2}$ as the test for x. Thus $$f(x)= 5x+10 \sin x \Rightarrow 5(\frac{\pi}{2}) + 10 \sin(\frac{\pi}{2})\Rightarrow \frac{5\pi}{2}+10$$
$\frac{5\pi}{2}+10 > 0$, so the function should be increasing at that point
The second interval test variable was $\pi$, and thus $5(\pi)+10 \sin(\pi)= 5\pi > 0$ So the interval of $(\frac{2\pi}{3}, \frac{4\pi}{3})$ should be increasing.
The third interval test variable was $\frac{3\pi}{2}$, and $5(\frac{3\pi}{2})+10 \sin(\frac{3\pi}{2})= \frac{15\pi}{2}-10 > 0$, so that interval is increasing as well.
However, the WebAssign site didn't accept those answers. What is missing with my logic and work?
Best Answer
You are nearly on the right track but are missing a key ingredient.
In calculus we learn that if the DERIVATIVE of a function is positive on an interval, then the function is increasing on that interval. If the DERIVATIVE of a function is negative on an interval, then the function is decreasing on that interval. So, you should first take the derivative to get $f'(x) = 5 + 10\cos x$ and then plug in values to the derivative (not the original function). That is, $f'(\pi/2) = 5 > 0$ so $f$ is increasing on $(0, 2\pi/3)$. $f'(\pi) = -5 < 0$, so $f$ is decreasing on the interval $(2\pi/3,4\pi/3)$. $f'(3\pi/2) = 5 > 0$, so $f$ is increasing on the third interval.
Now, there is also another missing step, which is to find the correct intervals to use. It seems as if you are given these intervals in the problem. If they were not given, or if you want to ensure you were given the correct intervals, you find where the derivative is 0 or undefined.