A professor of mathematics noticed that his kitchen sink at his home leaked. He called a plumber. The plumber came the next day and sealed a few screws, and eve

A professor of mathematics noticed that his kitchen sink at his home leaked. He called a plumber. 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. Well, all the same he paid it and then the plumber said to him, "I understand your position as a professor. Why don't you come to our company and apply for a plumber position? You will earn three times as much as a professor. But remember, when you apply, tell them that you completed only seven elementary classes. They don't like educated people." 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. One day, the board of the plumbing company decided that every plumber had to go to evening classes to complete the eighth grade. So, our professor had to go there too. It just happened that the first class was math. The evening teacher, to check students' knowledge, asked for a formula for the area of a circle. 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!!"