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### Course: Class 8 (Old) > Unit 8

Lesson 3: Multiplying monomials by polynomials# Multiplying monomials by polynomials challenge

Sal finds the values of coefficients c, d, and f that make -2y(y²+cy-3)=dy³+12y²+fy true for all y-values.

## Want to join the conversation?

- Can we solve for
`c`

algebraically?(20 votes)- Technically yes - however on its own, you would not reveal much. Remember that equations are about balancing. So if you isolated C =
*__*then that blank has to contain everything else in the equation.

So if you end up with something like "c = (dy^3 + 2y^3 12y^2 + fy - 6y) / 2y^2"

It doesn't really do you much good, since you can't simplify that much further and you still end up with multiple coefficients and exponents of y that you can't do anything with - even if you already know what c, d, and f equal.(4 votes)

- Why do we know that each of the terms are equal to their corresponding terms?(8 votes)
- I am late but...

In this equation we were always multiplying the same variable by a coefficient. Because the exponents and variables match, the only way that the problem would come out correct (within reason) would be if the coefficients also match.(2 votes)

- What would happen if it was (-2y^2+cy-3)(3 votes)
- Andy,

It would be the same. Just open the bracket and continue working.

Hope this helps(2 votes)

- hi my doubt is that whenever i try to solve questions like these i always go wrong in the answer like find a , b . i get the constant correct but i mess up in the sign of it like ist it positive or negative.need it asap(2 votes)
- Sounds like you need to practice multiplying signed numbers. The basic rules are:

positive * positive = positive

positive * negative = negative

negative * positive = negative

negative * negative = positive.

Note: You know you have an issue with getting the sign correct. So, when you think you are done multiplying, go back and recheck your signs. It takes very little effort, but finding and fixing the errors as you go will help you get a better answer.(3 votes)

- _ love this video, it helped me to better understand how to use the distributive properties(2 votes)
- 0:59I don’t get it . -2y and y^2 are not like terms so why do we have to combine them(1 vote)
- Combining like terms is what we do when we add / subtract. Yes, -2y and y^2 are unlike terms, so they can't be added/subtracted. But, the problem in the video is multiplication, not addition. We can always multiply: -2y(y^2) = -2y^3

Hope this helps.(2 votes)

- At1:56what is the meaning of "degree term"?(1 vote)
- You can learn about the degree of terms and polynomials in the intro to polynomials lesson: https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratics-multiplying-factoring/x2f8bb11595b61c86:multiply-monomial-polynomial/v/polynomials-intro(2 votes)

- After achieving the variables d, c, and f; Do you have to substitute the values into one of the polynomials to simplify the whole equation, or leave it as it is?(1 vote)
- I recall that matching up terms like that was something related the quadratic equation(1 vote)
- so what does the y do in this case?(1 vote)

## Video transcript

- [Voiceover] So we have
an equation right here that says negative two-y
times the expression y-squared plus c-y minus three is equal to d-y to the third
plus 12-y-squared plus f-y. And what I'd like you to do
is pause the video and see if you can figure out what
the variables c, d, and f are. Alright, so now let's work through it. So at first this could
look kind of intimidating. We just have an equation,
you're trying to figure out what these three variables
are, how do you do it? And one reasonable approach
would be let's just try to simplify what we have
here on the left hand side. And to simplify that
we can just distribute the negative two-y onto this polynomial, onto the y-squared plus c-y minus three. And then we could set it equal to what we have here on the right and let's see if we can match up coefficients. So let's distribute it. So first we could think about what negative two-y times
y-squared is going to be. Negative two-y times y-squared, well that's going to be negative
two-y to the third power. Cause y to the first times
y-squared is y to the third power Alright, now let's multiply
the negative two-y times c-y. Well that's gonna be negative two times c. So negative two-c and then
you're gonna have y times y. So negative two-c-y-squared. And then we would multiply
the negative two-y times, and we gotta keep in mind this is a, we're subtracting three here, so we could view that as a negative three. Negative two-y times negative three, negative two times negative
three is positive six, and we still have that y over there. So there you have it we have simplified the left hand side of this equation. And now let's see if, let
me actually just write the right hand side in different colors and then things might jump out at us. So over her I wrote this,
the third degree term, the y to the third term,
I wrote that in blue, so let me write the y to the
third term here in blue as well So d-y to the third. And then I wrote the
y-squared term in magenta so let me write the y-squared
term in magenta here. So plus 12-y-squared. And then last but not least I
wrote the first degree term, this y term in green, so let
me write the first degree term in green right over here, so plus f-y. And when you see it like
that you see which terms match up with (stutters). With which terms match
up to which other terms. So we could see that, look, I have the third degree term here. That has to match up to this
third degree term there. So d, d needs to be equal to negative two. So let's write that down, d is equal to negative two. We could see the second degree term. This second degree term matches up to this second degree
term right over there. So that tells us that
this coefficient, the 12, must be equal to the negative two-c. So let's write that down,
negative two-c is equal to 12. To solve for c we can divide
both sides by negative two. And we get c is equal to 12 divided by negative two is negative six. And that makes sense,
if c is negative six, negative two times negative six, or subtracting two times negative six, is subtracting negative 12, which would be the same
thing as adding 12. So then we get the exact same coefficient for the second degree term. I think you see where this is going. Over here we have a six-y
on the first degree term. Here we have an f-y on
the first degree term. This f must be the same thing, must be equal to the coefficient here. So f must be equal to six. And we're done. And the key realization
here is you match up the corresponding degree terms. There's no way that, well I don't want to get too complex here, but the simplest way to address this, is to realize, "Okay, I have
a third degree term here, "I have a third degree term here. "Well if I look at it very
simply the way I'm gonna "get this third degree term here "is using this third degree term." And I just look at the
coefficients and say, "Okay, they must have
the same coefficient." And then we say, "D is
equal to negative two", and then we keep doing that.