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

Here is my process.

$\displaystyle 7cos(7x)=0$

$\displaystyle cos(7x)=0$ divided the 0 by 7

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

$\displaystyle x=\frac{cos^{-1}(0)}{7}$

arc cos rules state the radian must be in [0,pi]

$\displaystyle arc cos(0) = \frac{pi}{2}$

so....

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

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

mathlab says the answer is $\displaystyle x=\frac{3pi}{14}$

what gives?