Well, talk about elementary differential equations. But all he uses is a simple harmonic motion. If the finding of the area of a circular sector exhausts the students' reservoir of thinking, then an understanding of the remainder of the proof is lost. The following method of proof avoids the use of area. The derivatives of the sine and cosine functions may be presented with the aid of vectors.
Velocity is perpendicular to the line of motion, etc.
Just a five-sentence article. A pie reposes on a plate of radius R . A piece of central angle is cut and put on a separate plate of radius r . Obviously r is a function of . There are sure discontinuities at 90 o and 180 o .