Explain xkcd: It's 'cause you're dumb.
Title text: Handy exam trick: when you know the answer but not the correct derivation, derive blindly forward from the givens and backward from the answer, and join the chains once the equations start looking similar. Sometimes the graders don't notice the seam.
In college courses with a very large number of students (picture the huge, tiered, amphitheater-style lecture halls shown in any movie or tv show about college), teaching assistants are employed to help the professors grade student work. In math and science cources, students are expected to solve the problems and show their work as supporting evidence. Due to the high volume of work to grade, whether it's being done by the professor or a TA, the grader will get lazy and look for correct answers and the existence of work without checking that the work is accurate.
[A problem is given on an arithmetic test: "4) 3x9=?". In handwriting, the student's work follows. The student has accurately reformatted the question as 3 times the square root of 81, which visually resembles the long division problem of 3 divided into 81, and then solved the latter to get 27--the correct answer to both.]
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In the middle of a Physics I exam, I forgot one of the equations of motion. Using my basic working knowledge of Calculus and the relationship between acceleration, velocity, and position, I managed to derive an equation which I used to solve the problem. When I got my exam back, I was given only partial credit because I got the right answer using the wrong formula.
I had an old math teacher once who didn't spend too much effort in grading trickier problems, so I got away with something similar in deriving Lagrange's Trig Identity in a complex class. Maybe 8 steps from the LHS and 2 steps from the RHS were right, and the equals sign that joined them was a leap of faith. --Quicksilver
) 01:55, 20 August 2013 (UTC)