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# Adding Odd Numbers

*We are very grateful to the Heilbronn Institute for Mathematical Research for their generous support for the development of this resource.*

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Age 11 to 16

Challenge Level

The sum of the first three odd numbers is $1+3+5 = 9$

**What are the first four odd numbers? What is the sum of the first four odd numbers?**

The first four odd numbers are $1$, $3$, $5$ and $7$. The sum of these is $1+3+5+7=16$.

What is the sum of the first five odd numbers?

What is the sum of the first two odd numbers?

What if we only have the first odd number in the sum?

What is the sum of the first six, or ten odd numbers?

Do you notice anything special about your results?

Can you predict what the sum of the first $100$ odd numbers will be?

The sum of the first $100$ odd numbers is $10,000$

**Can you predict what the sum of the first $n$ odd numbers will be?**

Mathematicians aren't usually satisfied with a few examples to convince themselves that something is always true, and look to proofs to provide rigorous and convincing arguments and justifications.

Can you prove that the sum of the first $n$ odd numbers is $n^2$?

Below is a proof that has been scrambled up.

Can you rearrange it into its original order?

Click on student solutions to see some different proofs that students submitted.

**Extension:**

Can you show diagrammatically that the sum of the first $n$ odd numbers is $n^2$?

You can try using Proof by Induction to prove the same result in the problem Adding Odd Numbers (part 2).