Factor 15x² + 66x + 51

Here we will show you how to factor the quadratic function 15x² + 66x + 51 using the box method. In other words, we will show you how to factor 15x squared plus 66x plus 51 (15x^2 + 66x + 51) using the box method. It is a 5-step process:

Step 1: The standard form of a quadratic equation is ax² + bx + c. We start by labeling the different parts of our equation 15x² + 66x + 51, like this:

a = 15
b = 66
c = 51

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

3	15x	51
3x	15x²	51x
	5x	17

We put 15x² (a) in the bottom left square and 51 (c) in the top right square, like this:

3	15x	51
3x	15x²	51x
	5x	17

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 15 times 51 (a × c), and add together to equal 66 (b).

More specifically, 15 times 51 is 765. Therefore, we need to find the two numbers that multiply to equal 765, and add to equal 66.

? × ? = 765
? + ? = 66

After looking at this problem, we can see that the two numbers that multiply together to equal 765, and add together to equal 66, are 15 and 51, as illustrated here:

15 × 51 = 765
15 + 51 = 66

Now, we can fill in the last two squares in our box with 15x and 51x. Place 15x in the upper left square, and place 51x in the lower right square.

3	15x	51
3x	15x²	51x
	5x	17

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 15x and 51. The greatest common factor of 15x and 51 is 3. Therefore, we write 3 to the left of the top row. You can see it here in the color green:

3	15x	51
3x	15x²	51x
	5x	17

Next, let’s look at the bottom row. We have the terms 15x² and 51x. The greatest common factor of 15x² and 51x is 3x. Therefore, we write 3x to the left of the bottom row. You can see it here in the color blue:

3	15x	51
3x	15x²	51x
	5x	17

To find the values below the table, we first divide 15x² by 3x (labeled in blue). This gives us 5x.

15x² ÷ 3x = 5x

You can see this value colored in orange below:

3	15x	51
3x	15x²	51x
	5x	17

Next, we divide 51x by 3x (labeled in blue). This gives us 17.

51x ÷ 3x = 17

You can see this value colored in purple below:

3	15x	51
3x	15x²	51x
	5x	17

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 15x² + 66x + 51. You simply put the values on the left in one set of parentheses, and the values below in another set of parentheses, to get the answer:

(3x + 3)(5x + 17)

That’s it! Now you know how to factor the equation 15x² + 66x + 51.

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