Question #21fcb

1 Answer
Jan 11, 2018

Proof below

Explanation:

We first use the cosine sum expansions:
cos(x-y)/cos(x+y)=cosxcosy+sinxsiny/cosxcosy-sinxsiny
Then divide top and bottom by cosxcosy
=((cosxcosy+sinxsiny)/(cosxcosy))/((cosxcosy-sinxsiny)/(cosxcosy))
And simplify
=(1+tanxtany)/(1-tanxtany)