4.1.28 For all integers n, if n is odd then n2 is odd.
Suppose that n is any odd integer. By definition of being odd, n = 2k +1, where k is some integer.
n2 = (2k+1)2
n2 = 4k2 + 4k + 1
Since 4k2 + 4k is even, 4k2+4k+1 is odd, thus n2 is od