# Half Angle formulas

**Half Angle formulas**?:

The Half angle formulas are stated below:

If ABC is a triangle , A , B and C are the three angles of the triangle and a , b , c are the sides opposite to the corresponding angles and

“s” is the semi perimeter or , , Then:

**Proof of Half angle formula**:

First of all let’s prove the half angle formula for

Using the cosine law:

Now using the trigonometric sub-multiple angle formula:

Now , let us prove the half angle formula for

Using the cosine law:

Lastly , Dividing by we get :

Similarly we can also prove the half angle formula of angle B and C.

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