Need the derivation of area formula of a segment

This is not in my syllabus. But on my geometry box i saw the formula.

**Radius(Pie Theta/360 - Sin Theta/2)**

We have area of segment in our syllabus but that consists of getting area of sector then subtracting the triangular area.

I tried to do some stuffs using that., using trignometry.

I first though Sin Theta gets us the Height of the triangle from the angle Theta. Like in 60 Degree it would be Root 3/2 but then i concluded its wrong.

Can anyone derive and explain the formula to me?

I am curious to know about it.