No, a semicircle is not a quadrilateral
The possessive form is semicircle's.
Calculating the radius of a semicircle depends on what information about the semicircle is given.
A semicircle means a half circle.
Semicircle has four syllables.
A semicircle has 180 degrees
A right angle triangle will fit into a semicircle.
The cross section of the Ram hemi is a semicircle
All triangles inscribed in a semicircle with one side of the triangle being the diameter of the semicircle are right triangles.
The diameter of the semicircle will be twice the radius.