SQUARING A NUMBER ENDING IN 5

Squaring A Number Ending In 5:

   A.  Squaring a number ending in 5 is very easy.  The method comes from algebra:

(10a + 5)2 = 100(a)(a + 1) + 25

   B.  Using numbers instead of variables we get the following:

1.  Write down 25.

2.  Multiply the number in the ten’s digit by that number plus 1. Write this number down.

Ex [1]  352 =_________.

a)      Write 25.

b)      3 x (3 + 1) = 3 x 4 = 12.  Write 12.

c)      The answer is 1225.

Ex [2]  1152 =_________.

a)      Think of 11 as being the number in the ten’s digit.

b)      Write 25.

c)      11 x (11 + 1) = 11 x 12 = 132.  Write 132.  

d)      The answer is 13225.