Derivative humor (William R Ward)
(smirk, math)
From the rec.humor.funny joke archives.

Paraphrased from joke told by Prof. Ed Landesman, University of California at Santa Cruz: Two math professors, Dr. Smith and Dr. Jones, are dining together in a restaurant. Dr. Smith is disillusioned about math teaching, and he says that he doesn't think people bother to learn what they teach, and that they shouldn't bother teaching math in the first place. Dr, Jones is trying to convince him otherwise, that people do listen in math class. They decide to settle their argument by asking their waiter a calculus question, and see if he gets the right answer. When Dr. Smith gets up to use the restroom, Jones calls the waiter over. Handing him a $20 bill, he says "When my colleague returns, I am going to ask you what the integral of ln x is, and you are to respond 1/x dx." The waiter agrees and walks off. When Smith gets back, they call the waiter over, and Jones asks him the question, which he answers correctly, much to Smith's surprise. As they walk out the door, Smith apologizing to Jones, the waiter calls out after them, "Plus a constant!"

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