Lines OA and OB is radius of circle.

Lines AB is one chords of circle.

The angle AOB is central angle.

Line OE is the apothem.

The curved line ADB is arc of the circle.

The area of a segment equals Sector Area - Triangle AOB Area

Line ED is the segment height .

The area AOBD is a sector.

Length of the arc ADB = (Central Angle / 180°) * π * r

Area of the sector AOBD = (Angle AOB / 360°) * π * r²

The ADBD area is the segment.