At the risk of sounding like a broken record, look for a set of identities written pretty much exactly in the form I have given. You won't find f() around anything, but you will find exactly the same mathematical transformation applied.
When you find that pair of identities, you can determine what f() is and make a circuit to do it.
This stuff is simple high school mathematics, and is one of the extremely simple cases that can be derived by inspection from the unit circle.
The other option is to go through that wiki page identity by identity until you find equations roughly in the form I gave you. To make it easy, look for all the identities that involve just sin and just cos, and have a change in sign of the angle on opposite sides of the identity. Look for all of them, because I can tell you that you will find a couple that won't work before you find the ones that obviously will.
