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### Course: Algebra (all content) > Unit 10

Lesson 8: Multiplying binomials by polynomials- Multiplying binomials by polynomials
- Multiply binomials by polynomials
- Multiplying binomials by polynomials: area model
- Multiplying binomials by polynomials challenge
- Multiplying binomials by polynomials review
- Multiplying binomials by polynomials (old)
- Multiplying binomials with radicals (old)

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# Multiplying binomials by polynomials challenge

Sal finds the values of coefficients a and b that make (2x+4)(5x-9)=ax²+bx-46 true for all x-values.

## Want to join the conversation?

- Why can't we just use a calculator?(0 votes)
- Because you have to burn your brain cells and pass the test.(25 votes)

- If the trinomial is in the front and the binomial comes afte, do you FOIL the binomial into the trinomial or do you FOIL the trinomial into the binomial?(4 votes)
- It doesn't matter. The commutative property of multiplication applies.

Remember: 2*3 is the same as 3*2.

This same concept applies when you multiply polynomials.

Hope this helps.(8 votes)

- At3:21what was all about the constant terms(4 votes)
- The constant terms are the ones that don't have any variables. If you do something we call the "Box method", they are the ones you should get in the bottom right. Again, they don't have a variable. No letter A-Z. I hope this helps you out!(0 votes)

- when multiplying polynomials and simplifying it. do i add the exponents -2mn(m2-4m-7n2) = -2m3n+8m2n+14mn2(2 votes)
- Yes, the properties of exponents tell us to add exponents when we are multiplying items with a common base.

FYI... write exponents correctly. you should have: -2m^3n+8m^2n+14mn^2

Without the symbols for exponents, you have -2m * 3n = -6mn(2 votes)

- What is the box method? Is it the same as FOIL?(1 vote)
- 3:00- There is no way to get the a and b values, except for a leap of logical faith and proving it afterwards?

I was trying to solve for a or b, but without a second equation, it seems impossibru.(1 vote)- In the video it shows (2x+4)(5x-9)

I'll do that equation.

you do foil(first, outer, inner last)

the layout is this: (dx + a) (cx + b)

(dx * cx) (first) + (dx*b) (outer) + (a*cx) (inner) + (a*b) last.

So you get (2x * 5x) (first) + (2x*-9) (outer) + (4*5x) (inner) + (4*-9) last.

You then get 10x^2 + -18x + 20x + -36

Which simplifies to 10x^2 + 2x - 36.

does this help?(2 votes)

- off topic but what are the energy points used for? because right know i have 18,311 points and i have no idea what to used them with(1 vote)
- I need help bad wth factoring polynomials test tomorrow(1 vote)

## Video transcript

- [Voiceover] So we've got
the expression two X plus four times five X minus nine
is equal to AX squared plus BX minus 36. And what we want to figure out is what are A and B going to be? And I encourage you to pause the video and try to figure it out. Well there's a coupe of
ways of trying to tackle it and the most straightforward would be just let's multiply these two binomials on the left-hand side
and let's see if we can match up the terms and
match up the coefficients. So let's multiply this left-hand side. There's a couple of ways to
think about tackling this. I like to think about it as applying the distributive property twice. So this expression on the left-hand side we can rewrite it, or one
way to think about it is we distribute the entire two X plus four onto the five X minus nine. So this is the same
thing as two X plus four times five X, plus two X plus four times negative nine. Or we could write it like this. It is five X five X times two X plus four. Two X plus four. And then we could either view this as a plus negative nine or just a minus nine. Minus nine times, once again, two X plus four. Two X plus four. And all we've done is we've distributed this two X plus four onto
the five X minus nine. Well now when we write it like this, when we look at the five X
times the two X plus four, we can distribute the five X. We can distribute the five
X onto the two X plus four. So what's five X times two X? Well that's going to be 10 X squared, five X times four is plus 20 X. Plus 20 X. And then we have, and then we have negative nine times two X is going to be negative 18 X. Negative 18 X. And then you have negative nine times four is negative 36. And now we can simplify this a little bit. We have two first degree terms. So let's see, we have 10 X squared, 10 X squared, and then these two first degree terms, let me circle them. So we have these two first degree terms. If I have 20 Xs and i were
to take away 18 of those Xs I'm going to have two X left over. 20 minus 18, two X. And then of course we still have the minus 36. Now all I've been doing so far is simplifying or rewriting
the left-hand side. We have to remember, this was an equation so this needs to be equal
to the right-hand side. So this is going to be
equal to AX squared. So AX squared plus BX, plus BX minus 36. Minus 36. And now that I've wrote it a little color-coded it
might jump out at you what A and B are going to be. We have 10 X squared over here and then the second degree
term on the right-hand side is AX squared. So 10 must be equal to A, or these two coefficients must be equal. So we could write A is equal to 10. And then when we look at
the first degree term, we have two X here and we
have BX right over here. And so two must be equal to B, or B must be equal to two. And it all worked out
that our constant terms are the same on both sides. So there we have it, A
equals 10, B equals two. Now once you're practiced at this you might be able to
say well how can I get a faster way to do this? Although it might be a
little bit more prone to careless mistakes. Is you could say well how can I get, how do I get an X squared? How do I, when I multiply
these things out, how do I get an X squared? Well the only way that
I can get an X squared is when I multiply the
two X times the five X. And that's going to be 10 X squared. And then you could say, alright, A is going to be equal to 10. And then you could say
how could I get an X? Well there's two ways that you can get, two ways to get an X. You could multiply two X times negative nine. So that would be negative 18 X. Or you could multiply four times five X, which is going to be plus 20 X. If you add these two
together you're going to get, they're going to be equal to two X. So two. Two X, BX, B must be equal to two. And then you can just check, well how am I gonna get a constant term? Well I have to multiply
these two constant terms. Four times negative nine,
you're gonna get negative 36. So the second way I just did it will be a little bit faster, you're a little bit more prone to making careless mistakes, but hopefully you appreciate that, I'm really just doing the same thing. Maybe with different levels of clarity.