Factor -49x² + 42x

Factor -49x² + 42x - 8

Here we will show you how to factor the quadratic function -49x² + 42x - 8 using the box method. In other words, we will show you how to factor negative 49x squared plus 42x minus 8 (-49x^2 + 42x - 8) 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 49x² - 42x + 8. Now we can label the different parts of our equation, like this:

a = 49
b = -42
c = 8

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

-4	-28x	8
7x	49x²	-14x
	7x	-2

We put 49x² (a) in the bottom left square and 8 (c) in the top right square, like this:

-4	-28x	8
7x	49x²	-14x
	7x	-2

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 49 times 8 (a × c), and add together to equal -42 (b).

More specifically, 49 times 8 is 392. Therefore, we need to find the two numbers that multiply to equal 392, and add to equal -42.

? × ? = 392
? + ? = -42

After looking at this problem, we can see that the two numbers that multiply together to equal 392, and add together to equal -42, are -28 and -14, as illustrated here:

-28 × -14 = 392
-28 + -14 = -42

Now, we can fill in the last two squares in our box with -28x and -14x. Place -28x in the upper left square, and place -14x in the lower right square.

-4	-28x	8
7x	49x²	-14x
	7x	-2

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 -28x and 8. The greatest common factor of -28x and 8 is -4. Therefore, we write -4 to the left of the top row. You can see it here in the color green:

-4	-28x	8
7x	49x²	-14x
	7x	-2

Next, let’s look at the bottom row. We have the terms 49x² and -14x. The greatest common factor of 49x² and -14x is 7x. Therefore, we write 7x to the left of the bottom row. You can see it here in the color blue:

-4	-28x	8
7x	49x²	-14x
	7x	-2

To find the values below the table, we first divide 49x² by 7x (labeled in blue). This gives us 7x.

49x² ÷ 7x = 7x

You can see this value colored in orange below:

-4	-28x	8
7x	49x²	-14x
	7x	-2

Next, we divide -14x by 7x (labeled in blue). This gives us -2.

-14x ÷ 7x = -2

You can see this value colored in purple below:

-4	-28x	8
7x	49x²	-14x
	7x	-2

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 -49x² + 42x - 8. You simply put the values on the left in one set of parentheses, and the values below in another set of parentheses, to get:

(7x - 4)(7x - 2)

In our original quadratic equation, -49x² + 42x - 8, a is negative. Therefore, we need to add a negative (-) sign before our two sets of parentheses, like this:

-(7x - 4)(7x - 2)

That’s it! Now you know how to factor the equation -49x² + 42x - 8.

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