Squares of Numbers That End in 5

A number that ends in 5 has the form A = 10a + 5, where a has one digit less than A. To find the square A² of A, append 25 to the product a×(a + 1) of a with its successor. For example, compute 115². 115 = 11·10 + 5, so that a = 11. First compute 11·(11 + 1) = 11·12 = 132 (since 3 = 1 + 2). Next, append 25 to the right of 132 to get 13225!

Why does this work?

 A²= (10a + 5)²
  = 100a² + 2·10a·5 + 25
  = 100a(a + 1) + 25.

Which exactly means that 25 is appended to the product a(a + 1).

Another example: to compute 245² observe that a = 24. Then

24·(24 + 1) = 24² + 24 = 576 + 24 = 600.

Therefore 245²=60025. Here is another way to compute 24·25:

24·25 = 2400/4 = 1200/2 = 600.

The rule naturally applies to 2-digit numbers as well. 75² = 5625 (since 7·8 = 56).


Related material
Read more...

  • Multiplication by 9, 99, 999, (Multiply + Subtract) etc.
  • Squaring 2-Digit Numbers
  • Division by 5
  • Multiplication by 2
  • Multiplication by 5
  • Multiplication by 9, 99, 999, etc. (Something Special)
  • Product of 10a + b and 10a + c where b + c = 10
  • Product of numbers close to 100
  • Product of two one-digit numbers greater than 5
  • Product of 2-digit numbers
  • Squaring Numbers in Range 26-50
  • Squaring Numbers in Range 51-100
  • Squares Can Be Computed Squentially
  • How to Compute Fast Any Square
  • Adding a Long List of Numbers

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