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## Get ready for Algebra 2

### Unit 1: Lesson 4

Multiplying binomials

# Multiplying binomials intro

Sal expresses (x-4)(x+7) as the standard trinomial x²+3x-28 and discusses how the general product (x+a)(x+b) can be written as x²+(a+b)x+a*b.

## Want to join the conversation?

• At he said "standard Quadratic form". Previously he spoke about standard from and just said you order in from greatest degree to lowest degree and didn't mention the Quadratic part, is standard Quadratic form more specific?? And what does the 'Quadratic' part explain?
• He's just specifying the type of equation he's working with.

Extra info: There are four specific ways to reference the degree (the greatest exponent) of an expression:

Linear: degree of 1; (x+2),(x)
Cubic: degree of 3; (x^3+x^2+x+1)
Quartic: degree of 4; (x^4)
It doesn't matter how many terms are in an expression when you're trying to determine its degree.

Information related to the kind of examples given: The number of terms in an expression has three specific words, and one general word.

Monomial: Only one term
Binomial: Two terms
Trinomial: Three terms
Polynomial (general): More than three terms
• Why plus seven at the start?
• I think the idea was that you take the two terms, here being x and positive 7 and distributing each term to (x-4) and once you get the binomial products, you add them together to get the answer of x^2+3x-28. Another method to multiply binomials is FOIL or First Outside Inside Last. Both of these methods will give you the same answers but FOIL is typically faster. Hope this helps!
• The general product can be written as shown above because he is basically taking steps instead of doing it at once .
• I distributed the (x+7) instead of distributing (x-4). Would that be wrong? or is the outcome the same all the time?
• The outcome would be the same as long as arrange the equation properly
• (x−3)(x−4)
I know how to do these kind of problems but what i don't understand is how to figure out what signs to use in the answer...
x^2-7x+12...
My question is where they would squeeze in an addition sign if its all subtraction...
• You need to review the rules for multiplying signed numbers.
A negative * a negative = a positive.
So, the +12 comes from -3 (-4) = +12
Hope this helps.
• What do you mean by degree??
• So, when multiplying binomials does it matter which one you multiply by? For example in this video could Sal have done x(x-7)-4(x-7)?
• What about multiplying binomials with two terms with numbers subtracted from the Xs? At , Sal finishes the only problem he solves in this video, which involves a st of both operations used in the Practice tab, but it is the only problem addressed in the video. Could someone pleas help me?
• Do you mean something like: (x-4)(x-7)?
You follow the same steps as in the video, except you'll have some different signs coming out of the multiplication.
(x-4)(x-7) = x(x-4)-7(x-4)
= x^2 - 4x - 7x + 28
= x^2 - 11x + 28

Hope this helped. If this wasn't what you were looking for, give a specific example.
(1 vote)
• At . Why say you are going to multiply it one way and then multiply it the totally opposite way? TOTAL OPPOSITE from the way you drew the arrows.
(1 vote)
• It's not the opposite way of multiplying it. He is actually doing exactly what he says. I can see why you think that, but what he is doing is multiply the number (x-4) by x and then multiplying (x-4) by 7 and adding the sum, which is exactly what he stated.