Let’s face it. Nobody actually knows how to derive proofs correctly. Scratch that…everyone knows how to derive proofs correctly, but never correct enough for the markers.
That’s right. You have a plethora of techniques to prove correctness of code or mathematical equations, but none of your techniques will be correct as far as the markers are concerned. This is obviously how it works in real life; when great physicists and mathematicians proved the correctness of their equations in their research papers, 15% of papers were rejected with comments and 85% were given a zero and a failing letter grade.
Today’s xkcd shows exactly what I mean. I had never thought of doing proofs in this manner before; rest assured, this is the approach I will take should I ever have to write another test on proofs.
EDIT: The caption text of that comic is: “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.”
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