I've been wondering about the theory behind the distributive property lately. For example: 2(pi * r^2) is just 2 * pi * r^2. However, when you add a positive number like +3. You get 2(PI*r^2 +3). But, that isn't just 2*PI*r^2+3. Its: 2pi*r^2 + 2*3. So I was wondering why that is. Why do you only have to multiply once with the whole multiplication part(with pi^r2), instead of having to multiply 2 by both pi and r^2. So then I thought isn't it just: (pi * r^2 + 3) + (pi * r^2 + 3)? Then, I tried to simplify that, thinking it would help me understand why...but all that did was make me more confused than when I started. Could someone help me understand please?

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    $\begingroup$ Please see here for a guide to writing math with MathJax, and see here for a guide to formatting posts with Markdown. $\endgroup$ – Zev Chonoles Aug 13 '13 at 2:43

The distributive property, and most basic properties of real numbers, comes from geometry.

A non-negative value corresponds to the length of a line segment.

Adding values corresponds to placing two segments together and measuring their length. Since the order that the segments are placed does not change the total length, addition is commutative. Looking at three segments, the length is the same if the first two are placed and then the third, or the last two are placed and then the first. Therefore addition is associative.

Multiplying two segments corresponds to getting the area of a rectangle with sides the lengths of the two segments. Since swapping the two segments just rotates the rectangle by 90 degrees, which does not change the area, multiplication is commutative.

Consider two rectangles with a common height $h$ and bases $a$ and $b$. Their areas are $ah$ and $bh$, and the sum of the areas is $ah+bh$. Place these two rectangles together so their common height lines up. They now form a single rectangle with base $a+b$ and height $h$, and the area of this rectangle is $(a+b)h$. This means that $ah+bh = (a+b)h$, which is the distributive law.

These laws were, of course, extended to other type of numbers (real, complex, fields, ...), but they all started with geometry.

Off topic but interesting: Try to prove that $\sqrt{2}$ is irrational using only geometric concepts and proofs. No algebra is allowed. (I think I'll even propose this as a question.)

  • $\begingroup$ I know we aren't supposed to post thank yous. But, wow, you opened up my mind. Thank you! $\endgroup$ – user2608474 Aug 13 '13 at 11:06
  • $\begingroup$ You are very welcome. $\endgroup$ – marty cohen Aug 13 '13 at 13:47

So one thing to be absolutely certain about is that the Distributive Property is only relevant when there is both addition(or subtraction) combined with a common multiplier (or divisor). For example:

3(x) = x + x + x = 3x

3(x + 1) = (x + 1) + (x + 1) + (x + 1) = 3x + 3

Now, as for your original question regarding 2(pi*r^2):

Notice that 2(pi*r^2) contains no addition or subtraction; therefore, the distributive property should not be used. When you want to add 3 to this expression, it starts to get tricky.

2(pi*r^2 + 3) = (pi*r^2 + 3) + (pi*r^2 + 3) = 2pi*r^2 + (3 + 3) = 2pi*r^2 + 6.

Hopefully this makes the use of the distributive property a little clearer. Again, the important thing to remember is that for the distributive property to apply, there needs to be both multiplication AND addition. If one or the other does not exist in your expression, then the distributive property does not come into play, and you have to deal instead with the commutative property as well as good old Order of Operations.


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