Explore properties of square roots and perfect squares.
a) Find all two-digit positive integers \(n\) for which \(4 < \sqrt{n} < 5\).
b) Square each digit from \(0\) through \(9\). Which digits can appear in the ones place of a perfect square?
c) Use your result from part b) to explain without a calculator why \(157\) cannot be the square of a positive integer.
Hints
- Relate the square-root inequality to an inequality involving squares.
- When squaring a number, focus on its ones digit.
- Which digits do not appear in your list from part b)?
Solution
1. The inequality \(4 < \sqrt{n} < 5\) is equivalent to \(4^2 < n < 5^2\), so \(16 < n < 25\). Thus, \(n\) can be \(17, 18, 19, 20, 21, 22, 23\), or \(24\).
2. The digit squares are \(0, 1, 4, 9, 16, 25, 36, 49, 64\), and \(81\). Their ones digits are \(0, 1, 4, 5, 6\), and \(9\).
3. Every positive-integer square ends in \(0, 1, 4, 5, 6\), or \(9\). Since \(157\) ends in \(7\), it cannot be a perfect square.
Answer
a) \(n \in \{17, 18, 19, 20, 21, 22, 23, 24\}\)
b) The possible ones digits are \(0, 1, 4, 5, 6\), and \(9\).
c) The number \(157\) ends in \(7\), but no positive-integer square ends in \(7\). Therefore, \(157\) is not a perfect square.