**This is the third in the Variations on Numbers Series.**

Three is the only prime which is one less than a perfect square: 4 – 1 = 3

All you need is three points to define a plane, or flat surface.

Any fraction with three in the denominator will end up having repeating threes or sixes, forever. Like 2/3 = 0.66666… or 7/3 = 2.3333….

The song “Three Is a Magic Number” is a catchy tune to help remember the multiplication facts for 3s.

### Checking Divisibility by Three

You can check to see if the number is divisible by three by adding up the digits of that number. If the digits are divisible by three, the original number is!

For example the number 547,842 is divisible by three: 5+4+7+8+4+2 = 30

If we check the number 4,679,58,231,795,544,122, the digits add to 84. We can tell that 84 is divisible by three because 8 + 4 = 12 is divisible by three. Therefore that original giant crazy number is divisible by three.

### What’s your variation on three?

Where do you see the number 3 in your world? And what are the variations?

Share your thoughts and experiences in the comments, and don’t forget to talk about the number 3 with your children!

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A 3-sided polygon, a triangle, is the only “rigid” shape. Any polygon with more than 3 sides is not rigid because it can be easily deformed. A triangle, by contrast, is strong because it resists deformation. Therefore, triangles are often used in construction. For example, “trusses” are structures made of steel beams in the form of triangles. Often, many smaller triangles are welded together to form larger triangles for greater strength. Trusses are used wherever strength is needed: bridges and supports of all kinds. The Eiffel Tower is made of trusses consisting of thousands of steel triangle-shaped constructions.

Very cool, Allen! Reading that gave me goosebumps (circular, but they don’t need to be rigid!)