X^2=5^2+8^2-2(5)(8)(cos(55))

Ineed to learn how to solve this problem

There were 816 people at a concert when a band started to play. after each song, only half the people stayed to hear the next song. how many people were left at the concert after x songs? write a function to represent this scenario.

For which pair of functions is (gof)(a)=|a|-2?