1. The problem statement, all variables and given/known data cos^-1(sin(5pi/4)) 2. Relevant equations n/a 3. The attempt at a solution I began by finding the reference angle of 5pi/4 by subtracting pi, and I got pi/4. pi/4 must be part of an isosceles right triangle, so I drew a diagram with sides 1,1, and √2. I found sin pi/4 to equal 1/√2, which I rationalized and got √2/2. I viewed my cosine chart, and cosine is √2/2 where the angle is pi/4. I answered pi/4 into MyMathLab, which found it incorrect, so I concluded that I am most likely doing it wrong.