28Jun/105
3×9
by Jeff
Image 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.
Alright, this math equation only works because 9 is the square of 3, but this is fun none the less.
3 x 9 = 3 x the square root of 81 (the square root of 81 is 9).
That also equals 81 divided by 3 and then he solves the division by hand to get 27.
This is obviously not the way you solve the problem, but he uses handy math tricks like turning a square root into a division symbol to attempt to trick the math grader as he mentions in the image text.
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June 29th, 2010
Especially funny, since 3×9 is very basic multiplication, too simple to even need to show your work.
June 29th, 2010
Great observation there! I couldn’t figure out the reason as to why this 3 x 9 worked. As you pointed out, you can do this with any situation where you are multiplying a number by it’s square root. For example, 1 x 1, 2 x 4, 3 x 9, 4 x 16, 5 x 25, etc.
I can now let go of my headache of trying to figure out the patter now
June 30th, 2010
Thanks for the explanation. I suspected it was something like this, but I have never used or seen the “square root” notation for long division so I didn’t get the details.
June 30th, 2010
Yeah, that’s not how I learned to write divisions either, so I didn’t get the joke. I know it’s hard but you should try to be more internationally-aware (is that a word ?) when explaining stuff.
June 30th, 2010
I took the joke portion to be something slightly different as in, the transposing of similar symbols.
In this case the joke being, the person taking the test doesn’t know the answer to the question so they try putting the equation in different forms,
3 x 9= ? = 3 x SR81 = 3(SR81) <— this being the important part,
as removing the multiplication sign "x" makes the equation 3(SR81) which looks visually similar to elementary school division where 3(SR81) looks like 3 divided into 81, which because 9 is the square of 3 just happens to work.