Question
Write five numbers which you cannot decide just by looking at their unit’s digit (or one’s place) whether they are square numbers or not.
Solution
Any natural number ending in 0, 1, 4, 5, 6 or 9 can be or cannot be a square-number. Five such numbers are:
56790, 3671, 2454, 76555, 69209
Property 2. If a number has 1 or 9 in the unit’s place, then its square ends in 1.
For example:
(1)2 = 1, (9)2 = 81, (11)2 = 121, (19)2 = 361, (21)2 = 441.
