Q. Find the squares of the following numbers containing 5 in unit’s place.
(i) 15
Assume a number with unit digit 5 = a5
= = 10a(10a + 5) + 5(10a + 5) = 100a 2 + 50a + 50a + 25
= 100a(a + 1) + 25
= a(a + 1) hundred + 25
We will use this result here,
We have a5 = 15 , So a = 1
= 1(1+1)100 + 25 = 200 + 25 = 225