In Exercises 6366, powers of trigonometric functions are
Chapter 5, Problem 5.2.63(choose chapter or problem)
In Exercises 6366, powers of trigonometric functions are rewritten to be useful in calculus. Verify the identity.
Unfortunately, we don't have that question answered yet. But you can get it answered in just 5 hours by Logging in or Becoming a subscriber.
Becoming a subscriber
Or look for another answer