Triangular Numbers: Sum from 1 to N

Kitten and I covered triangular numbers a couple months ago in our Competition Math for Middle School book, but I think it’s time to revisit the topic. I like the method James Tanton gives in this new video:

How Do You Count Infinity?

Tanton continues to play with triangular numbers in the next video, which is probably too abstract for Kitten, but I enjoyed it. The triangular numbers are countably infinite, as are the square numbers. But if you look at it another way…


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