Hi, I have the following problem.

"Prove that for any positive integers a, b, c, d satisfying

ab-cd=1,

no cancellation is possible in the fraction

(a+c)/(b+d) (where the result of the fraction is a rational number."

Well, I know how to cancel a fraction, but how am I supposed to do this one, and what letters should I use to prove the cancellation?

I'd really appreciate it if you could give me a hand, thanks.