Factor -11x² - 91x

Factor -11x² - 91x - 80

Here we will show you how to factor the quadratic function -11x² - 91x - 80 using the box method. In other words, we will show you how to factor negative 11x squared minus 91x minus 80 (-11x^2 - 91x - 80) using the box method. It is a 5-step process:

Step 1: The standard form of a quadratic equation is ax² + bx + c. In this equation, a is negative. Therefore, we need to start by setting aside the negative (-). We do this by flipping the signs in the equation to get 11x² + 91x + 80. Now we can label the different parts of our equation, like this:

a = 11
b = 91
c = 80

Step 2: Next, we need to draw a box and divide it into four squares:

1	11x	80
x	11x²	80x
	11x	80

We put 11x² (a) in the bottom left square and 80 (c) in the top right square, like this:

1	11x	80
x	11x²	80x
	11x	80

Step 3: In order to fill in the other two squares, we need to do some math. We have to find two numbers that multiply together to give us the product of 11 times 80 (a × c), and add together to equal 91 (b).

More specifically, 11 times 80 is 880. Therefore, we need to find the two numbers that multiply to equal 880, and add to equal 91.

? × ? = 880
? + ? = 91

After looking at this problem, we can see that the two numbers that multiply together to equal 880, and add together to equal 91, are 11 and 80, as illustrated here:

11 × 80 = 880
11 + 80 = 91

Now, we can fill in the last two squares in our box with 11x and 80x. Place 11x in the upper left square, and place 80x in the lower right square.

1	11x	80
x	11x²	80x
	11x	80

Step 4: Next, we need to find four final numbers to finish factoring our equation. We can see that our box is a 2 x 2 table made up of rows and columns.

Our goal is to find the numbers that, when multiplied together, result in the products in each square. We can do this by finding the greatest common factor in each row.

Let’s look at the top row. We have the terms 11x and 80. The greatest common factor of 11x and 80 is 1. Therefore, we write 1 to the left of the top row. You can see it here in the color green:

1	11x	80
x	11x²	80x
	11x	80

Next, let’s look at the bottom row. We have the terms 11x² and 80x. The greatest common factor of 11x² and 80x is x. Therefore, we write x to the left of the bottom row. You can see it here in the color blue:

1	11x	80
x	11x²	80x
	11x	80

To find the values below the table, we first divide 11x² by x (labeled in blue). This gives us 11x.

11x² ÷ x = 11x

You can see this value colored in orange below:

1	11x	80
x	11x²	80x
	11x	80

Next, we divide 80x by x (labeled in blue). This gives us 80.

80x ÷ x = 80

You can see this value colored in purple below:

1	11x	80
x	11x²	80x
	11x	80

Step 5: Now we have all of the information we need to finish factoring the equation. The values outside of the box are used to factor -11x² - 91x - 80. You simply put the values on the left in one set of parentheses, and the values below in another set of parentheses, to get:

(x + 1)(11x + 80)

In our original quadratic equation, -11x² - 91x - 80, a is negative. Therefore, we need to add a negative (-) sign before our two sets of parentheses, like this:

-(x + 1)(11x + 80)

That’s it! Now you know how to factor the equation -11x² - 91x - 80.

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