Åk 6–9

8.4 Distributive Laws
Distributive laws connect multiplication with addition and subtraction. We usually call these “multiplying in” or “breaking out”.

We begin with an example:

You know that 3 · 9 = 27

We can divide 9 into two terms, for example (5 + 4).

If we multiply 3 by (5 + 4), will we get the same answer as if we had multiplied 3 · 9? What do you think?

We can illustrate this in pictures: Here you see that we get the same result, 27.

You can try to divide 9 into other terms and multiply by 3. Check and see if you get the same results.

In you mathematics book, it may be explained like this: 3(5 + 4) = (3 · 5) + (3
· 4) = 15 + 12 = 27

When you calculated with written multiplication, you might use the distributive laws without being aware of it.

Here is an example when you use it:

 2 2 · 5 1 1 1 0

 5 · 22 = 5 · (2 + 20) = = 5 · 2 + 5 · 20 = = 10 + 100 = 110

Now you have learned to “multiply in”, and now we can show you how to “break out”.

15 + 12 has a common factor, 3.

15 can be written (5 · 3) and 12 can be written (4 · 3).  We can then write the addition as follows:

 15 + 12 = 27 (5 · 3) + (4 · 3) = 27

We break out the common factor 3:  You have learned that the distributive law applies for multiplication with addition in parenthesis. Does it work even for multiplication with subtraction in parenthesis?  We will look at an example:

5
· (8 - 6)

Multiply in:
5 · (8 - 6) = (5
· 8) - (5
· 6) = 40 - 30 = 10

Calculate the parenthesis first:
5 · (8 - 6) = 5
· 2 = 10

We get the same answer and the rule seems to apply no matter if it´s multiplication with addition or subtraction in the parenthesis.

We can summarise the calculation rule for multiplication with addition or subtraction in the parenthesis in something called the distributive law: and   