# Divisibility Rules for 3 and 9 (VIDEO!)

Most everyone seems to know the rule for divisibility by three: add up the digits and see if the sum is a multiple of three. Most seem to “know” that the same rule holds for nine.

But why do these rules work? And did you know that summing the digits tells you a bit more than just a yes/no answer?

Here is all the math explained – swiftly and easily.

Divisibility_3_and_9

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Written for educators - and their students too! - this website, slowly growing, takes all the content Tanton has developed in his books, videos, and workshops, and organizes it into short, self-contained, and complete, curriculum units proving that mathematics, at all points of the school curriculum, can be joyous, fresh, innovative, rich, deep-thinking, and devoid of any rote doing! Let's teach generations of students to be self-reliant thinkers, willing to flail and to use their common sense to "nut their way" through challenges, to assess and judge results, and to adjust actions to find success. (Great life skills!)

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