Chapter 1 - Section 1.8 - Proof Methods and Strategy - Supplementary Exercises - Page 113: 42

$18$ is one of the positive integers for which, we can say that this can be written as the sum of squares of positive integers in two different ways.

The squares of positive integers are $1, 4, 9, 16, 25, ……$ We have to Note that $18$ can be written as the two different sums of squares of positive integers: $18=9+9$ $18=4+4+4+4+1+1 $ Thus $18$ is a positive integer for which the statement is true.