Amusing math trick. Grab a calculator. (you won't be able to do this one in your Head) 1. Key in the first three digits of your phone number (NOT the Area code) 2. Multiply by 80 3. Add 1 4. Multiply by 250 5. Add the last 4 digits of your phone number 6. Add the last 4 digits of your phone number again. 7. Subtract 250 8. Divide number by 2 Do you recognize the answer ?? 
What do you mean I can't do that in my head? Speak for yourself. 
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thats cool! 
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(((80*abc + 1)*250)+2*defg250)/2 = (80*250*abc + 250 +2*defg250)/2 = (20000*abc +2*defg)/2 = 10000*abc + defg = abc * 10^4 + defg = abc0000 + defg = abcdefg = your phone number There's nothing interesting about this. A gradeschooler could invent the steps to get this result, as it's all based on (80 *250)/2 = 10000. 
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Would punching 58,008 into your calculator and then turning it upside down qualify as a math trick? 
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If you take a convolution integral, you can LaPlace transform the arguments, add them together, and inverse LaPlace transform them, and the result is the product of the convolution arguments. Now that's a sweet math trick. $ 
My favorite math trick so far is either using the heat equation to value a future option or using modular arithmetic on large numbers to encode a message into an uncrackable code (RSA). 
Pointing out to mav fans that 3 > 0 is another math trick I find quite entertaining. Quote:

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Did you know that 142.857 is a curious number? 142.857 x 2 = 285.714 ( same digits ) 142.857 x 3 = 428.571 idem 142.857 x 4 = 571.428 idem 142.857 x 5 = 714.285 idem 142.857 x 6 = 857.142 idem 142.857 x 7 = 999.999 !!! 142.857 x 8 = 1.142.856 (number missing is 7, but last 6 and first 1 add to 7 142.857 x 9 = 1.285.713 (number missing is 4, but last and first add to 4 ) You can go on and on... :elefant :D 
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This one is the key to all that. Multiples of 9 produce a lot of odd number things. Remember the way the whole 9 times table added up to 9? 9 x 1 = 09 ; 9 + 0 = 9 9 x 2 = 18 ; 1 + 8 = 9 9 x 3 = 27 ; 2 + 7 = 9 9 x 4 = 36 ; 3 + 6 = 9 9 x 5 = 45 ; 4 + 5 = 9 9 x 6 = 54 ; 5 + 4 = 9 9 x 7 = 63 ; 6 + 3 = 9 9 x 8 = 72 ; 7 + 2 = 9 9 x 9 = 81 ; 8 + 1 = 9 9 x 10 = 90 ; 9 + 0 = 9 :elefant 

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