# Thread: set function = 0

1. ## set function = 0

I have to find $7cos(7x)=0$ and solve for X. I thought I had it correct, but Mathlab says otherwise.

Here is my process.

$7cos(7x)=0$
$cos(7x)=0$ divided the 0 by 7
$7x=cos^{-1}(0)$ take the Arc-Cos of both sides
$x=\frac{cos^{-1}(0)}{7}$

arc cos rules state the radian must be in [0,pi]
$arc cos(0) = \frac{pi}{2}$
so....

$x=\frac{\frac{pi}{2}}{7}$

$x=\frac{pi}{14}$

mathlab says the answer is $x=\frac{3pi}{14}$
what gives?

2. ## Re: set function = 0

I forgot to add critical info. The interval is $(\frac{pi}{7},\frac{2pi}{7})$

But i still am not sure.

3. ## Re: set function = 0

what is the period of the function cos(7x) (that is, how often do the values repeat)?

clearly π/14 isn't in the specified interval. what is?