Recently, I made actual use of the Pythagorean Theorem. I was replacing a light fixture in my family room that fit into a square hole 13 inches on a side. I was looking at some circular fixtures, and wanted to know what diameter it had to be to cover the hole. 13 squared is 169, 169 +169 = 338, the square root of 338 = 18.38. That is just under 50 cm, and I was able to find a fixture 50 cm in diameter. It covers the hole nicely. The salesman said that he hadn't even thought of the Pythagorean Theorem since high school.