
Doug C. answered 04/29/21
Math Tutor with Reputation to make difficult concepts understandable
Do you know how to factor x4 - y4?
That is a difference of squares, so it factors like this:
(x2 - y2)(x2 + y2).
Do the same for sin4(α) - cos4(α).
Then realize that there is a Pythagorean Trigonometric Identity that allows you to replace sin2(α) + cos2(α) with the value 1. From there simplify and combine similar terms and you will have your answer.