I'm practicing trig and so I'm working through some exercises. I'm having some trouble figuring this one out:

First I've rewritten as which equates to:

.

Then I'm kind of stuck, so I tried going the other way around:

=

Then I multiply the 1/1 in the denominator by cos a cos b, to make them equal, to get rid of the 1, then I rewrite it according to = which leaves me with a lot of sin and cos, but stuck as well. Some help would be nice