# Sum to Product identity of Sine functions

## Formula

$\sin{\alpha}+\sin{\beta}$ $\,=\,$ $2\sin{\Big(\dfrac{\alpha+\beta}{2}\Big)}\cos{\Big(\dfrac{\alpha-\beta}{2}\Big)}$

An identity that expresses the transformation of sum of sine functions into product form is called the sum to product identity of sine functions.

### Introduction

Let $\alpha$ and $\beta$ be two angles of right triangles. The sine functions with the two angles are written as $\sin{\alpha}$ and $\sin{\beta}$ mathematically. The sum of the two sine functions is written mathematically in the following form.

$\sin{\alpha}+\sin{\beta}$

The sum of sine functions can be transformed into the product of the trigonometric functions as follows.

$\implies$ $\sin{\alpha}+\sin{\beta}$ $\,=\,$ $2\sin{\Big(\dfrac{\alpha+\beta}{2}\Big)}\cos{\Big(\dfrac{\alpha-\beta}{2}\Big)}$

#### Other forms

The sum to product transformation rule of sin functions is popular written in two forms.

$(1). \,\,\,$ $\sin{x}+\sin{y}$ $\,=\,$ $2\sin{\Big(\dfrac{x+y}{2}\Big)}\cos{\Big(\dfrac{x-y}{2}\Big)}$

$(2). \,\,\,$ $\sin{C}+\sin{D}$ $\,=\,$ $2\sin{\Big(\dfrac{C+D}{2}\Big)}\cos{\Big(\dfrac{C-D}{2}\Big)}$

In the same way, you can write the sum to product transformation formula of sine functions in terms of any two angles.

#### Proof

Learn how to prove the sum to product transformation identity of sine functions in trigonometry.

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