# How can you prove that 1+5+9+⋯+(4n−3)=2n2−n1+ 5+ 9 + \cdots +(4n-3) = 2n^{2} – n without using induction?

Using mathematical induction, I have proved that

for every integer $n > 0$.

I would like to know if there is another way of proving this result without using PMI. Is there any geometric solution to prove this problem? Also, are there examples of problems where only PMI is our only option?

Here is the way I have solved this using PMI.

Base Case: since $1 = 2 · 1^2 − 1$, the formula holds for $n = 1$.

Assuming that the
formula holds for some integer $k ≥ 1$, that is,

I show that

Now if I use hypothesis I observe.

$\diamond$