Measure of an Angle in a Semi-Circle

This GeoGebra example demonstrates that any angle inscribed in a semicircle, with the diameter as one side, is always a right angle (90°). The core mathematical concept is the inscribed angle theorem and its corollary: an angle subtended by a diameter is a right angle. It involves angle measurement, circle properties, and geometric reasoning. Construction details: using GeoGebra commands Segment to create diameter AB, Semicircle to draw the semicircle, Point to place a movable point C on the arc, Textfield and InputBox1 to allow user input for a guessed angle value, and text1 to display the measured angle. By dragging point C, learners can observe that the angle remains 90° regardless of position. This resource is suitable for middle school geometry lessons or high school review of circle theorems, and supports inquiry-based learning to verify the constancy of the angle in a semicircle.