Triangular numbers

Hello again! Today let's do some more sums:
こんにちは！　今日はもう一度数を足しましょう：

1 = 1
1 + 2 = ...
1 + 2 + 3 = ...
1 + 2 + 3 + 4 = ...
1 + 2 + 3 + 4 + 5 = ...
etc.

The answers to these sums are called 'triangular numbers', because they are the numbers of balls we can put in the shape of a triangle:
この合計の答えの数は、ボールで作った三角の形のボールの数なので、＜三角数＞（さんかくすう）という:

Next time we'll see how to calculate triangular numbers more quickly! See you then :-)
次回は、三角数をもっと早く計算できる方法を分かります。　お楽しみに！