Odd number sums

Let's start with some adding exercises. Try adding up odd numbers:
最初は奇数を足そう。　これをやって見ましょう:
1 = 1
1 + 3 = ...
1 + 3 + 5 = ...
1 + 3 + 5 + 7 = ...
1 + 3 + 5 + 7 + 9 = ...
1 + 3 + 5 + 7 + 9 + 11 = ...
1 + 3 + 5 + 7 + 9 + 11 + 13 = ...
etc.

Can you see the pattern? :-)
どうなっているのが分かりましたか？