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Distributive property over addition

Learn how to apply the distributive law of multiplication over addition and why it works. This is sometimes just called the distributive law or the distributive property. Created by Sal Khan and Monterey Institute for Technology and Education.

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Video transcript

Rewrite the expression 4 times, and then in parentheses we have 8 plus 3, using the distributive law of multiplication over addition. Then simplify the expression. So let's just try to solve this or evaluate this expression, then we'll talk a little bit about the distributive law of multiplication over addition, usually just called the distributive law. So we have 4 times 8 plus 8 plus 3. Now there's two ways to do it. Normally, when you have parentheses, your inclination is, well, let me just evaluate what's in the parentheses first and then worry about what's outside of the parentheses, and we can do that fairly easily here. We can evaluate what 8 plus 3 is. 8 plus 3 is 11. So if we do that-- let me do that in this direction. So if we do that, we get 4 times, and in parentheses we have an 11. 8 plus 3 is 11, and then this is going to be equal to-- well, 4 times 11 is just 44, so you can evaluate it that way. But they want us to use the distributive law of multiplication. We did not use the distributive law just now. We just evaluated the expression. We used the parentheses first, then multiplied by 4. In the distributive law, we multiply by 4 first. And it's called the distributive law because you distribute the 4, and we're going to think about what that means. So in the distributive law, what this will become, it'll become 4 times 8 plus 4 times 3, and we're going to think about why that is in a second. So this is going to be equal to 4 times 8 plus 4 times 3. A lot of people's first instinct is just to multiply the 4 times the 8, but no! You have to distribute the 4. You have to multiply it times the 8 and times the 3. This is right here. This is the distributive property in action right here. Distributive property in action. And then when you evaluate it-- and I'm going to show you in kind of a visual way why this works. But then when you evaluate it, 4 times 8-- I'll do this in a different color-- 4 times 8 is 32, and then so we have 32 plus 4 times 3. 4 times 3 is 12 and 32 plus 12 is equal to 44. That is also equal to 44, so you can get it either way. But when they want us to use the distributive law, you'd distribute the 4 first. Now let's think about why that happens. Let's visualize just what 8 plus 3 is. Let me draw eight of something. So one, two, three, four, five, six, seven, eight, right? And then we're going to add to that three of something, of maybe the same thing. One, two, three. So you can imagine this is what we have inside of the parentheses. We have 8 circles plus 3 circles. Now, when we're multiplying this whole thing, this whole thing times 4, what does that mean? Well, that means we're just going to add this to itself four times. Let me do that with a copy and paste. Copy and paste. Let me copy and then let me paste. There you go. That's two. That's one, two, three, and then we have four, and we're going to add them all together. So this is literally what? Four times, right? Let me go back to the drawing tool. We have it one, two, three, four times this expression, which is 8 plus 3. Now, what is this thing over here? If you were to count all of this stuff, you would get 44. But what is this thing over here? Well, that's 8 added to itself four times. You could imagine you're adding all of these. So what's 8 added to itself four times? That is 4 times 8. So this is 4 times 8, and what is this over here in the orange? We have one, two, three, four times. Well, each time we have three. So it's 4 times this right here. This right here is 4 times 3. So you see why the distributive property works. If you do 4 times 8 plus 3, you have to multiply-- when you, I guess you could imagine, duplicate the thing four times, both the 8 and the 3 is getting duplicated four times or it's being added to itself four times, and that's why we distribute the 4.