BEATCALC:  Beat the Calculator!

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Back to Calculation Tips & Tricks
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From the BEATCALC mailing list:
a weekly service from B. Lee Clay, archived here by the Math Forum.
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   Squaring special numbers (9's and final 2)

  1. Choose a number with repeating 9's and a final 2.
  2. The square is made up of:
    • one fewer 9 than there are repeating 9's
    • 84
    • the same number of 0's as there are 9's in the square
    • A final 64

   Example:

  1. If the number to be squared is 9992:
  2. The square has:

    one fewer 9 than the
       repeating 9's:         9 9
    84                            8 4
    same number of 0's as 9's
       in the square                  0 0
    a final 64                            6 4

  3. So 9992 x 9992 = 99840064.

   See the pattern?

  1. If the number to be squared is 999992:
  2. The square has:

    one fewer 9 than the
       repeating 9's:      9 9 9 9
    84                             8 4
    same number of 0's as 9's
       in the square                   0 0 0 0
    a final 64                                 6 4

  3. So 999992 x 999992 = 999984000064.

With a little practice you will be finding these huge products with ease.


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13 October 1997
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