The plumber came the next day and sealed a few screws, and everything was working as before.The professor was delighted. However, when the plumber gave him the bill a minute later, he was shocked.
“This is one-third of my monthly salary!” he yelled.
So it happened. The professor got a job as a plumber and his life significantly improved. He just had to seal a screw or two occasionally, and his salary went up significantly.
The person asked was the professor. He jumped to the board, and then he realized that he had forgotten the formula. He started to reason it, and he filled the white board with integrals, differentials, and other advanced formulas to conclude the result he forgot. As a result, he got “minus pi times r square.”
He didn’t like the minus, so he started all over again. He got the minus again. No matter how many times he tried, he always got a minus. He was frustrated. He gave the class a frightened look and saw all the plumbers whisper: “Switch the limits of the integral!”