Question

What is the cosine of 13pi/6 and what other radian does it corresponds to?

Answer 1

`2pi = (12pi)/6` corresponds to a full circle
so

`(13pi)/2 = (12pi)/6 +pi/6`

is equivalent to `pi/6`

Depending upon what you mean by "corresponds to"
`cos(x) = cos(-x)`

so
`cos((13pi)/6) = cos(pi/6) = cos(-pi/6)`

and in addition any angle which can be obtained from either of these by adding or subtracting an integer multiple of `2pi` will also be equivalent.

`cos(pi/6)` is the `cos` of a standard right angled triangle with sides `1, sqrt(3), 2`

`cos((13pi)/6) = sqrt(3)/2`