Factor -87x² + 86x + 40

Here we will show you how to factor the quadratic function -87x² + 86x + 40 using the box method. In other words, we will show you how to factor negative 87x squared plus 86x plus 40 (-87x^2 + 86x + 40) 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 87x² - 86x - 40. Now we can label the different parts of our equation, like this:

a = 87
b = -86
c = -40

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

-4	-116x	-40
3x	87x²	30x
	29x	10

We put 87x² (a) in the bottom left square and -40 (c) in the top right square, like this:

-4	-116x	-40
3x	87x²	30x
	29x	10

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 87 times -40 (a × c), and add together to equal -86 (b).

More specifically, 87 times -40 is -3480. Therefore, we need to find the two numbers that multiply to equal -3480, and add to equal -86.

? × ? = -3480
? + ? = -86

After looking at this problem, we can see that the two numbers that multiply together to equal -3480, and add together to equal -86, are -116 and 30, as illustrated here:

-116 × 30 = -3480
-116 + 30 = -86

Now, we can fill in the last two squares in our box with -116x and 30x. Place -116x in the upper left square, and place 30x in the lower right square.

-4	-116x	-40
3x	87x²	30x
	29x	10

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 -116x and -40. The greatest common factor of -116x and -40 is -4. Therefore, we write -4 to the left of the top row. You can see it here in the color green:

-4	-116x	-40
3x	87x²	30x
	29x	10

Next, let’s look at the bottom row. We have the terms 87x² and 30x. The greatest common factor of 87x² and 30x is 3x. Therefore, we write 3x to the left of the bottom row. You can see it here in the color blue:

-4	-116x	-40
3x	87x²	30x
	29x	10

To find the values below the table, we first divide 87x² by 3x (labeled in blue). This gives us 29x.

87x² ÷ 3x = 29x

You can see this value colored in orange below:

-4	-116x	-40
3x	87x²	30x
	29x	10

Next, we divide 30x by 3x (labeled in blue). This gives us 10.

30x ÷ 3x = 10

You can see this value colored in purple below:

-4	-116x	-40
3x	87x²	30x
	29x	10

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 -87x² + 86x + 40. You simply put the values on the left in one set of parentheses, and the values below in another set of parentheses, to get:

(3x - 4)(29x + 10)

In our original quadratic equation, -87x² + 86x + 40, a is negative. Therefore, we need to add a negative (-) sign before our two sets of parentheses, like this:

-(3x - 4)(29x + 10)

That’s it! Now you know how to factor the equation -87x² + 86x + 40.

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