need help w/proof cos(a-b)= (cosA*cosB)+(sinA*sinB)

Name:
E-Mail:
Subject:
Comments:	: Can someone help me prove this identity? : I have some trig. teaching software but all it says is that the distance formula, for the chord that seperates the angles can be used to prove it. It's important because it form the basis for many other identities use in integration techniques. Such as the double angle and product identities. : Thank a bunch folks. : This wil help with an up and coming Diff EQ class in case I forget the formulas.
Optional Link URL:
Link Title:
Optional Image URL: