Compound Angle Formulae - Introduction

In this topic, we learn about addition formula or compound angle formula. These formulas are applied when a trig function has two angles or letters inside the bracket. It cannot be expanded as sin (A+B), so we need to use special formulas given in the formula book. For example, sin (A+B) = sin A cos B + cos A sin B. We need to memorize these formulas and know how to use them. There are two main applications of addition formulas in questions. The first one is to solve questions without using a calculator by expressing them as combinations of known angles. The second one is to prove identities containing sin, cos, or tan with triangles inside. There are three formulas for plus signs and three for minus signs, but they are interchangeable. The goal is to manipulate these formulas to prove certain identities.