This property of circles was discovered by the Great Greek Mathematician named Thales about 2500 year ago. This is called the inscribed angle subtended by the semicircle is always a right angle. I have explain it below;
Following are the links to my previous lessons on the circle geometry;
https://steemit.com/math/@mathworksheets/circle-geometry-lesson-plan-1
https://steemit.com/math/@mathworksheets/circle-geometry-lesson-3-properties-of-a-chord
https://steemit.com/math/@mathworksheets/circle-geometry-central-angle-and-inscribed-angle