Multiplying Polynomials 08:42 minutes

Video Transcript

Transcript Multiplying Polynomials

”Stuck in her tower, watchin' the hours pass,
MC Prince's gonna be put on blast.
For makin' Rap wait, that ain't cool,
ensconced in the tower like a fool,
With no tools, no hope and no measure,
an empty room gives a girl no pleasure”

”Her hands, her thumbs 'n that hair is what she'll apply
to the situation she needs to rectify,
...to deck out her room with some fly gear
b'fore she goes crazy like King Lear
For a sick new room to make her feel jovial,
Rap's gotta know how to multiply polynomials”

Defining the unkown quantities

Let's help Rap-Punzel measure her room using her long locks as a measuring tool. The dimensions of her rectangular room are eight times the length of the section of her hair that she uses to measure. The length of her hair is an unknown quantity; we’ll call this 'x'. The length of her room is 40 times the width of her hand. The width of her hand is another unknown quantity, which we'll call 'y'.So, the width of the room is 8x, and the length of the room is 40y.
Do you remember how to calculate the area of a rectangle? Of course you do! You multiply the length times the width. To figure out the area of Rap-Punzel's room, multiply the two monomials. 8x(40y). 8(40) = 320, and x(y) = xy, so the product of the two monomials is 320xy. Next, Rap-Punzel measures the closet. The length is 16 times the width of her hand, or 16y. The width is 1 times the length of her hair plus 2 times the width of her hand, giving us the binomial x + 2y.

Multiplying Polynomials

Again, find the area by multiplying, but this time you multiply a monomial by a binomial. Let’s work this out. To do this, you can use the Distributive Property. Remember the Distributive Property? a(b+c) = ab + ac? You can apply this property to numbers, variables and polynomials. For this expression, a = 16y, b = x, and, c = 2y.
Now solve by distributing the 16y to the two terms that are summed inside the parentheses, so the resulting binomial is 16xy + 32y². You can also set this expression up using an area model created with one row and 2 columns. The row is labeled 16y, and 2 columns are labeled x and 2y to model the x plus 2y. Multiply the row times each column to calculate the area of each of the rectangle's sections. The area of the first rectangle is 16xy, and the area of the second is 32y². Add them together and you get the same answer as before, 16xy + 32y².
Rap-Punzel uses the width of her hand and also her thumb to measure the length and width of her magical mirror. The length of the mirror is 8 times her hand and 7 times her thumb. She knows her thumb is about an inch wide. The width then, is 6 times her hand and 5 times her thumb.

FOIL Method

We can use a special method to calculate the area of the magical mirror. This method, the FOIL method, works only when you multiply two binomials. To FOIL, multiply the first two terms of each binomial. Next, multiply the outer terms of each binomial. Then, multiply the inner terms and last, multiply the last terms. Check it out!? It kind of looks like a claw! Let’s try it out using the measurements of the mirror. First, 8y(6y) = 48y². Outer, 8y(5) = 40y. Inner, 7(6y) = 42y and Last, 7(5) = 35. Combine the like terms and rewrite the terms in the standard form, listing the exponents in order from greatest to least. 48y² + 82y + 35.

Area Model

There's another method to solve this. You can also use the area model. This time, the area model is set up using 4 rectangles, to represent each of the four terms in the two binomials. The four rectangles are arranged in 2 rows and 2 columns. To find the products, multiply the rows by the columns to find each area, add the like terms, and write the result in standard form. Using the area model, what's the product of the two binomials? Just the same as before: 48y² + 82y + 35.

Multiplying trinomials by binomials

Lastly, Rap-Punzel measures her bed. The width of the bed is 2 hair lengths minus 2 hand widths plus 3 thumbs, and the length is 7 hand widths plus 5 thumbs. A trinomial times a binomial?! What can Rap-Punzel do to figure out the area of the bed? She can’t FOIL because you only use the FOIL method to multiply two binomials, but she can use the always-dependable Distributive Property.
Let’s distribute! Again, use the Distributive Property as your guide to rewrite the expression. Multiply the trinomial by the first term of the binomial, 7y, and then multiply the trinomial by the second term of the binomial, 5. Now add both of the results together. As always, write it in standard form. You can also use the area model; this time, set it up with 3 rows and 2 columns. Watch out for the negative signs when you calculate the area for each rectangular section and also when you combine like terms. Just like before, the product of the two polynomials is -14y² + 14xy + 11y + 10x + 15.

”With her room all done and er'thing in its place,
Rap moves the mirror to reveal a space,
A hole in the wall, what a trip
time to escape and begin her courtship,
of Prince MC, oh where can he be?”

In the tower, that's where...eh, c'est la vie!