Perfect Square

The term perfect square is used in mathematics in two meanings:
  • a positive integer which is the square of some other integer, i.e. can be written in the form n2 for some integer n.
  • an algebraic expression that can be factored as the square of some other expression, e.g. a2 ± 2ab + b2 = (a ± b)2. (see Square (algebra)).
This is not the same as a magic square.

Using differences of squares as multiplication

Integer multiplication can be done entirely by a difference of two squares. Examples:
  • 10\times 10 = 10^2 - 0^2 = 100 - 0 = 100
  • 9\times 11 = 10^2 - 1^2 = 100 - 1 = 99
  • 8\times 12 = 10^2 - 2^2 = 100 - 4 = 96
  • 7\times 13 = 10^2 - 3^2 = 100 - 9 = 91
In general, the product of two numbers is equal to the square of their average minus their difference from the average squared. A geometric constructive "proof" of this relation is shown the following animation: The starting rectangle is A by B. The resulting large square is length (A+B)/2, and the smaller gray square (remainder being subtracted) is length |A-B|/2. Using this relation, you can multiply relatively large nearly equal numbers more quickly if you memorize a relatively small list of squares. If you're multiplying an even by an odd, you can avoid "halves" by adjust one number, by requiring one more addition at the end
  • A\times B = A\times (B-1) + A
Example:
  • 27\times 34 = 33 + 27 = - 3^2 + 27 = 900 - 9 + 27 = 918

 

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