Finding values between two mins or two maxes

Hello,

I am trying to find the value between two maxes/two mins. I just need to see how long a wavelength is.

I have the equation. It is a cos function: 2 + 2.5cos(2pi*60*t).

So, In order to find all maxes and mins, I will derive the above function, and I get:

-sin(2pi*60*t)

I need to set this to zero to find the x values of the max and mins. This is where the problem comes in. I have forgotten how to solve for this:

0 = -sin(2pi*60*t)

I do the inverse of sin of both sides and get this:

0 = -(2pi*60*t)

What is the inverse sin of 0? It's a large range correct?

The next max or min can be found by doing:

pi = -(2pi*60*t) correct?

I tried that, and I get 0 and 1/30, which is def. not the interval between a max and a min. I was going to find the time between a max and a min and multiply by two to find the length of the wavelength. When I calculated this eq. into my graphing calculator, 1/15 is def. not the distance of a wavelength.

Thanks.