Conversation so far:
what is the diameter of a semi circle
A semicircle's straight side is it's diameter.
What is the area of a semi circle?
The area of a semicircle is half the area area of the circle from which it is made.
how is a semicircle used in math
A semicircle can be used to construct the arithmetic and geometric means of two lengths using straight-edge and compass.
what is the angle of the arc of the semicircle
The full arc of a semicircle always measures 180°

Question: what is the symmetry of the shape
Self-contained rewrite: what is the symmetry of the semi circle