Most bodies will apparently bethink the times tables from primary academy quizzes. There adeptness be patterns in some of them (the simple acceleration of the 2 times table) but others you aloof learnt by rote. And it was never absolutely bright aloof why it was all-important to apperceive what 7 x 9 is off the top of your head.

Well, accept no fear, there will be no cardinal quizzes here. Instead, I appetite to appearance you a way to body numbers that gives them some structure, and how multiplication uses that structure.

Multiplication artlessly gives you the breadth of a rectangle, if you apperceive the lengths of the sides. Aces any aboveboard in the grid, (for example, let’s aces the 7th access in the 5th row) and colour a rectangle from that aboveboard to the top larboard corner.

A rectangle of admeasurement 5 × 7 in the multiplication table.

This rectangle has breadth 7 and acme 5, and the breadth (the cardinal of blooming squares) is begin in the dejected amphitheater in the basal appropriate corner! This is accurate no amount which brace of numbers in the filigree you pick.

Now let’s booty this rectangle and cast it about the capital askew (the red dotted line).

The aforementioned rectangle, flipped.

The breadth and acme of the rectangle accept swapped, but the breadth hasn’t changed. So from this we can see that 5 × 7 is the aforementioned as 7 × 5. This holds accurate for any brace of numbers — in mathematics we say that multiplication is commutative.

But this actuality agency that there is a agreement in the multiplication table. The numbers aloft the askew band are like a mirror angel of the numbers beneath the line.

So if your aim is to memorise the table, you absolutely alone charge to memorise about bisected of it.

To go added with multiplication we aboriginal charge to do some dividing. Bethink that adding a cardinal aloof agency breaking it into pieces of according size.

12 ÷ 3 = 4

This agency 12 can be burst into 3 pieces, anniversary of admeasurement 4.

Since 3 and 4 are both accomplished numbers, they are alleged factors of 12, and 12 is said to be divisible by 3 and by 4. If a cardinal is alone divisible by itself and 1, it is alleged a prime number.

But there’s added than one way to address 12 as a artefact of two numbers:

12 × 1

6 × 2

4 × 3

3 × 4

2 × 6

1 × 12

In fact, we can see this if we attending at the multiplication table.

The occurrences of 12 in the multiplication table.

The cardinal of coloured squares in this account tells you there are six agency you can accomplish a rectangle of breadth 12 with accomplished cardinal ancillary lengths. So it’s additionally the cardinal of agency you can address 12 as a artefact of two numbers.

Incidentally, you adeptness accept noticed that the coloured squares assume to anatomy a bland ambit — they do! The ambit aing the squares is accepted as a hypera, accustomed by the blueprint a × b = 12, area ‘a’ and ‘b’ are not necessarily accomplished numbers.

Let’s attending afresh at the account of articles aloft that are according to 12. Every cardinal listed there is a agency of 12. What if we attending at factors of factors? Any agency that is not prime (except for 1) can be breach into added factors, for example

12 = 6 × 2 = (2 × 3) × 2

12 = 4 × 3 = (2 × 2) × 3

No amount how we do it, back we breach the factors until we’re larboard alone with primes, we consistently end up with two 2’s and one 3.

This product

2 × 2 × 3

is alleged the prime atomization of 12 and is altered to that number. There is alone one way to address a cardinal as a artefact of primes, and anniversary artefact of primes gives a altered number. In mathematics this is accepted as the Axiological Theorem of Arithmetic.

The prime atomization tells us important things about a number, in a actual abridged way.

For example, from the prime atomization 12 = 2 × 2 × 3, we can see anon that 12 is divisible by 2 and 3, and not by any added prime (such as 5 or 7). We can additionally see that it’s divisible by the artefact of any best of two 2’s and one 3 that you appetite to pick.

Furthermore, any assorted of 12 will additionally be divisible by the aforementioned numbers. Consider 11 x 12 = 132. This aftereffect is additionally divisible by 1, 2, 3, 4, 6 and 12, aloof like 12. Adding anniversary of these with the agency of 11, we acquisition that 132 is additionally divisible by 11, 22, 33, 44, 66 and 132.

It’s additionally accessible to see if a cardinal is the aboveboard of addition number: In that case there charge be an alike cardinal of anniversary prime factor. For example, 36 = 2 × 2 × 3 × 3, so it’s the aboveboard of 2 × 3 = 6.

The prime atomization can additionally accomplish multiplication easier. If you don’t apperceive the acknowledgment to 11 × 12, again alive the prime atomization of 12 agency you can assignment through the multiplication footfall by step.

11 x 12

= 11 x 2 × 2 × 3

= ((11 x 2) × 2) × 3

= (22 × 2) × 3

= 44 × 3

= 132

If the primes of the atomization are baby abundant (say 2, 3 or 5), multiplication is nice and easy, if a bit paper-consuming. Thus adding by 4 (= 2 x 2), 6 (= 2 x 3), 8 (= 2 x 2 x 2), or 9 (= 3 x 3) doesn’t charge to be a alarming task!

For example, if you can’t bethink the 9 times table, it doesn’t amount as continued as you can accumulate by 3 twice. (However this adjustment doesn’t advice with adding by beyond primes, actuality new methods are appropriate – if you haven’t apparent the ambush for the 11 times tables watch this video).

So the adeptness to breach numbers into their prime factors can accomplish complicated multiplications abundant simpler, and it’s alike added advantageous for bigger numbers.

For example, the prime atomization of 756 is 2 x 2 x 3 x 3 x 3 x 7, so adding by 756 artlessly agency adding by anniversary of these almost baby primes. (Of course, award the prime atomization of a ample cardinal is usually actual difficult, so it’s alone advantageous if you already apperceive what the atomization is.)

But added than this, prime decompositions accord axiological advice about numbers. This advice is broadly advantageous in mathematics and added fields such as cryptography and internet security. It additionally leads to some hasty patterns – to see this, try colouring all multiples of 12 in the times table and see what happens. I’ll leave that for homework.

Anita Ponsaing is Research Associate in Mathematics at University of Melbourne.

This commodity was originally appear on The Conversation. Read the aboriginal article. Angel by Tiger Pixel beneath Creative Commons license.

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