Can somebody help with this question, thank you:)

If tana tanb=k, show that(1+k)cos(a+b)=(1-k)cos(a-b)

The functions below show the amount of money bella and sweet t had saved after earning money for doing chores. which description best compares the two functions?

In a singing competition, there are 150 participants. at the end of each round, 40% of the participants are eliminated. how many participants are left after n rounds?