The problem invites conjecture and trying to prove the conjecture.
It can be proved using the sum of the natural numbers (an AP) or by
mathematical induction.
Possible approach
The teacher might leave the students to try their own methods or
suggest mathematical induction if the class would beneft from
practice in that technique or alternatively suggest that they use
the formula for the sum of the natural numbers if she wants the
class to have practice in simple algebraic manipulation.
Key question
What do we already know that we can use for this proof?