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## Multiplying monomials by polynomials

Current time:0:00Total duration:5:17

# Area model for multiplying polynomials with negative terms

## Video transcript

- [Instructor] In previous videos, we've already looked at using area models to think about multiplying expressions, like multiplying x plus
seven times x plus three. In those videos, we saw
that we could think about it as finding the area of a rectangle, where we could break up
the length of the rectangle as part of the length has length x, and then the rest of it has length seven. So this would be seven here,
and then the total length of this side would be x plus seven. And then the total length of
this side would be x plus, and then you have three right over here. And what area models did
is they helped us visualize why we multiply the different terms or how we multiply the different terms. Because if we're looking
for the entire area, the entire area is going
to be x plus seven, x plus seven times x plus three, times x plus three. And then of course, we can break that down into these sub-rectangles. This rectangle, and this is
actually going to be a square, would have an area of x squared. This one over here will
have an area of seven x, seven times x. This one over here will
have an area of three x. And then this one over
here will have an area of three times seven, or 21. And so we can figure out that
the ultimate product here is going to be x squared plus seven x plus three x plus 21. That's going to be the area
of the entire rectangle. Of course, we could add the seven x to the three x to get to 10x. But some of you might be wondering, well, this is all nice when I have plus seven and plus three. I can think about positive lengths. I can think about positive areas. But what if it wasn't that way? What if we were dealing
with negatives instead? For example, if we now
try to do the same thing, we could say, all right, this top length right over here would be x minus seven. So let's just keep going with it, and let's call this length
negative seven up here. So it has a negative seven
length, and we're not necessarily used to thinking
about lengths as negative. Let's just go with it. And then the height right over here, it would be x minus three. So we could write an x there
for that part of the height. And for this part of the height, we could put a negative three. So let's see, if we kept going
with what we did last time, the area here would be x squared. The area here would be
negative seven times x, so that would be negative seven x. This green area would be negative three x. And then this orange area
would be negative three times negative seven,
which is positive 21. And then we would say that the
entire product is x squared minus seven x minus three x plus 21. And we can, of course, add these two together
to get negative 10x. But does this make sense? Well, one way to think about
it is that a negative area is an area that you would
take away from the total area. So if x happens to be
a positive number here, then this pink area would be negative, and so we would take
it away from the whole. And that's exactly what is
happening in this expression. And it's worth mentioning that even before when this
wasn't a negative seven, when it was a positive seven and this was a positive seven x, it's completely possible
that x is negative, in which case you would've
had a negative area anyway. But to appreciate that
this will all work out, even with negative numbers, I'll give an example,
if x were equal to 10. That will help us make sense of things. So if x were equal to 10, we would get an area model
that looks like this. We're having 10 minus seven, so I'll put minus seven right over here, times 10 minus three. Now, you can figure out in your heads what's that going to be. 10 minus seven is three. 10 minus three is seven. So this should all add up to positive 21. Let's make sure that's actually occurring. So this blue area is going to
be 10 times 10, which is 100. This pink area now is
10 times negative seven. So it's negative 70, so we're gonna take it
away from the total area. This green area is
negative three times 10, so that's negative 30. And then negative three
times negative seven, this orange area is positive 21. Does that all work out? Let's see, if we take this
positive area, 100 minus 70 minus 30 and then add 21, 100 minus 70 is going to be 30, minus 30 again is zero, and then you just have 21 left over, which is exactly what you would expect. You could actually move
this pink area over and subtract it from this blue area. And then you could take this green area and then you could turn it vertical, and then that would subtract
out the rest of the blue area. And then all you would have
left is this orange area. So hopefully this helps you appreciate that area models for multiplying
expressions also works if the terms are negative. And also, reminder, when
we just had x's here, they could've been negative to begin with.