Take the three sides of RT triangle ABC as the diameter and make three semicircles outwards. What is the quantitative relationship between their areas S1, S2 and S3?

Take the three sides of RT triangle ABC as the diameter and make three semicircles outwards. What is the quantitative relationship between their areas S1, S2 and S3?

If S3 is the area of a circle with the diameter of the hypotenuse, then S1 + S2 = S3