# Homework Help: Strange Trig Function solutions

1. May 19, 2007

### WolfOfTheSteps

1. The problem statement, all variables and given/known data

This is part of a problem for a nonlinear diff class... But it's the basic stuff that's tripping me up.

Find all the max/min and concavity for

$$v(x) = -cos(x)-Lx+1,\ \ \ 0<L<1$$

3. The attempt at a solution

Here's what I do:

$$v'(x) = sin(x)-L$$
$$v''(x)=cos(x)$$

Set the first derivative to 0:

$$sin(x)-L=0 \Rightarrow x = arcsin(L)$$

Here's where I'm confused. I say

$$x = arcsin(L) + 2\pi n, \ \ \ \mbox{where }n \mbox{ is any integer}$$

But Maple says:

$$x = arcsin(L)+2\pi n, \ \ \ \mbox{where }n \mbox{ is any integer, OR:}$$
$$x = arcsin(L) - 2arcsin(L)+2\pi n+\pi \ \ \ \mbox{where }n \mbox{ is any integer}$$

Where does the -2arcsin(L) and the +pi come from?????

I really want to understand this once and for all.... I never took a trig class, and while I get by fine 99% of the time, I hit a brick wall when I come across this kind of stuff.

Thanks!

2. May 19, 2007

### WolfOfTheSteps

I think I almost understand it... The second set of values of x are the ones where the arcsin is in the II quadrant. Correct?

And obviously it makes more sense to write:

$$x = \pi - arcsin(L) + 2\pi n$$

for the 2nd set of x values.

Yeah, I think I get it now. I was just looking at the graph of arcsine, and it doesn't seem to make sense from that.

Is there a way to see that those are the solutions by looking at the arcsine graph?

Thanks.

3. May 19, 2007

### arunbg

I hope you understand the first solution.
The second part of the solution is better understood if you manipulate the terms a bit , it can be written as $x = (2n+1)\pi-arcsin(L)$.
Noting that $sin(n\pi-x)=sin(x)$ for all odd n, you can now see how this is also part of the solution.

If you still don't follow, perhaps substitution of values for L into the solutions can give a clearer picture.
For the graphical picture, you should use the sin(x) graph. Fix some L value, find arcsin(L), and see what other values of x, do you get sin(x)=L, that's all.

Last edited: May 19, 2007