We'll use this identity:
cos(90 - x) = sin(x)
where x is any real number
If x = 90, then sin(x) = sin(90) = 1 which is given to us.
So this means
cos(90 - x) = sin(x)
cos(90 - 90) = sin(90)
cos(0) = 1
So it's likely your teacher is looking for you to write cos(0) = 1
However, the prompt and instructions are a bit vague. There are other possibilities that I may not be thinking of.
