##### Rivkah Carl, I work at Noodle

This is a difficult question to think about. But here are some suggestions as to how you can begin to think and verify the equation. 1) Think about the popular trigonometric identity cos^2(x) + sin^2(x)=1. 2) Think about the relationship between cos(x) and cos(x+pi). Choose a value for x and test it out. 3) Think about the relationship between sin and cos. Make note of whether or not they are positive or negative in different quadrants. 3) Try watching other videos about trigonometric identities until you get a better understanding Here are a couple more: https://www.noodle.com/learn/details/305223/trigonometric-identities https://www.noodle.com/learn/details/150919/trigonometric-identities https://www.noodle.com/learn/details/339944/verifying-a-trigonometric-identity-establish

Good luck!