We know that given any whole number n, 2n + 1 represents an odd number.
We also know that multiplying any number by 2 results in an even number and an even number added to an odd number results in an odd number.
Proof:
Is it true when n = 1?
1 x 3 = 3
Yes, it is true for n = 1
Assume true for n = k; that is: (k)(k+2) is an odd number.
Under this assumption, will it be true for k+1?
[k+1][(k+1) + 2] = [k + 1][k + 3] = k² + 4k + 4
= k² + 2k + 2k + 4 = (k)(k + 2) + 2(k + 2)
Since, by assumption, (k)(k+2) is an odd number, and 2(k + 2) is an even number (because it is a number multiplied by 2), then (k)(k + 2) + 2(k + 2) is an odd number (because it is an odd number added to an even number).
Yes, it is true under the assumption.
Q.E.D.