ao and ob are called radius

ac, bc and ab are called strings

angle acb is the angle of the circle

angle aob is the center image of the circle

the curved part ab is arc.

oe is the edge-center distance

triangle aob is a triangle area

ed is the arch height

the arc length calculation formula is: (central angle / 180°) •π • r

the yellow area is fan-shaped

the formula for sector area is: (angle aob / 360°) •π • r²

the green area is the arch height area

the size of this area = sector area - aob triangle