SAT Math : How to factor an equation

Study concepts, example questions & explanations for SAT Math

varsity tutors app store varsity tutors android store varsity tutors ibooks store

Example Questions

Example Question #1 : Factoring Equations

Factor the following equation. 

x– 16

Possible Answers:

(x + 4)(x + 4)

(x – 4)(x – 4)

(x2)(4 – 2) 

(x)(x – 4)

(x + 4)(x – 4)

Correct answer:

(x + 4)(x – 4)

Explanation:

The correct answer is (x + 4)(x – 4) 

We neen to factor x– 16 to solve. We know that each parenthesis will contain an x to make the x2. We know that the root of 16 is 4 and since it is negative and no value of x is present we can tell that one 4 must be positive and the other negative. If we work it from the multiple choice answers we will see that when multiplying it out we get x+ 4x – 4x – 16. 4x – 4x cancels out and we are left with our answer. 

Example Question #3 : Equations / Solution Sets

If x3 – y3 = 30, and x2 + xy + y2 = 6, then what is x2 – 2xy + y2?

Possible Answers:

5

25

24

cannot be determined

180

Correct answer:

25

Explanation:

First, let's factor x3 – y3 using the formula for difference of cubes.

x3 – y= (x – y)(x2 + xy + y2)

We are told that x2 + xy + y2 = 6. Thus, we can substitute 6 into the above equation and solve for x – y.

(x - y)(6) = 30.

Divide both sides by 6.

x – y = 5.

The original questions asks us to find x2 – 2xy + y2. Notice that if we factor x2 – 2xy + y2 using the formula for perfect squares, we obtain the following:

x2 – 2xy + y= (x – y)2.

Since we know that (x – y) = 5, (x – y)2 must equal 52, or 25.

Thus, x2 – 2xy + y= 25.

The answer is 25.

Example Question #1 : How To Factor An Equation

if x – y = 4 and x2 – y = 34, what is x?

Possible Answers:

10

12

9

15

6

Correct answer:

6

Explanation:

This can be solved by substitution and factoring.

x2 – y = 34 can be written as y = x2 – 34 and substituted into the other equation: x – y = 4 which leads to x – x2 + 34 = 4 which can be written as x2 – x – 30 = 0.

x2 – x – 30 = 0 can be factored to (x – 6)(x + 5) = 0 so x = 6 and –5 and because only 6 is a possible answer, it is the correct choice.

Example Question #2 : How To Factor An Equation

If x2 + 2ax + 81 = 0.  When a = 9, what is the value of x?

Possible Answers:

–18

–9

9

0

3

Correct answer:

–9

Explanation:

When a = 9, then x2 + 2ax + 81 = 0 becomes 

x2 + 18x + 81 = 0.  

This equation can be factored as (x + 9)2 = 0.

Therefore when a = 9, x = –9.

Example Question #3 : How To Factor An Equation

If f(x) has roots at x = –1, 0 and 2, which of the following could be the equation for f(x)?

Possible Answers:

f(x) = x2 + x – 2

f(x) = x– x – 2

f(x) = x– x– 2x

f(x) = x– x+ 2x

f(x) = x4 + x3 – 2x2

Correct answer:

f(x) = x– x– 2x

Explanation:

In general, if a function has a root at x = r, then (x – r) must be a factor of f(x). In this problem, we are told that f(x) has roots at –1, 0 and 2. This means that the following are all factors of f(x):

(x – (–1)) = x + 1

(x – 0) = x

and (x – 2).

This means that we must look for an equation for f(x) that has the factors (+ 1), x, and (x – 2).

We can immediately eliminate the function f(x) = x2 + x – 2, because we cannot factor an x out of this polynomial. For the same reason, we can eliminate f(x) = x– x – 2.

Let's look at the function f(x) = x– x+ 2x. When we factor this, we are left with x(x2 – x + 2). We cannot factor this polynomial any further. Thus, x + 1 and x – 2 are not factors of this function, so it can't be the answer.

Next, let's examine f(x) = x4 + x3 – 2x.

We can factor out x2.

x(x+ x – 2)

When we factor x+ x – 2, we will get (x + 2)(x – 1). These factors are not the same as x – 2 and x + 1. 

The only function with the right factors is f(x) = x– x– 2x.

When we factor out an x, we get (x– x – 2), which then factors into (x – 2)(+ 1). Thus, this function has all of the factors we need.

The answer is f(x) = x– x– 2x.

Example Question #2 : Factoring Equations

Factor 36x2 – 49y2.

Possible Answers:

(6x – 7y)(6x – 7y)

(6x + 7y)(6x – 7y)

6x2 – 7y2

cannot be factored

(6x + 7y)(6x + 7y)

Correct answer:

(6x + 7y)(6x – 7y)

Explanation:

This is a difference of squares. The difference of squares formula is a2 – b2 = (a + b)(a – b). In this problem, a = 6x and b = 7y.

So 36x2 – 49y= (6x + 7y)(6x – 7y).

Example Question #8 : Equations / Inequalities

Solve for x:

\dpi{100} \small x^{2}-2x-48 = 0\(\displaystyle \dpi{100} \small x^{2}-2x-48 = 0\)

Possible Answers:

\dpi{100} \small x=8,-6\(\displaystyle \dpi{100} \small x=8,-6\)

Correct answer:

\dpi{100} \small x=8,-6\(\displaystyle \dpi{100} \small x=8,-6\)

Explanation:

Find two numbers that add to \dpi{100} \small -2\(\displaystyle \dpi{100} \small -2\) and multiply to \dpi{100} \small -48\(\displaystyle \dpi{100} \small -48\)

Factors of \dpi{100} \small 48\(\displaystyle \dpi{100} \small 48\)

\dpi{100} \small 1,2,3,4,6,8,12,16,24,48\(\displaystyle \dpi{100} \small 1,2,3,4,6,8,12,16,24,48\)

You can use \dpi{100} \small -8 +6 =-2\(\displaystyle \dpi{100} \small -8 +6 =-2\)

\dpi{100} \small (x-8)(x+6) = 0\(\displaystyle \dpi{100} \small (x-8)(x+6) = 0\)

Then make each factor equal 0.

\dpi{100} \small x-8 = 0\(\displaystyle \dpi{100} \small x-8 = 0\) and \dpi{100} \small x+6 = 0\(\displaystyle \dpi{100} \small x+6 = 0\)

\dpi{100} \small x= 8\(\displaystyle \dpi{100} \small x= 8\) and \dpi{100} \small x=-6\(\displaystyle \dpi{100} \small x=-6\)

Example Question #1 : Equations / Inequalities

Find the roots of f(x)=x^2+2x-3\(\displaystyle f(x)=x^2+2x-3\)

Possible Answers:

x=0,3\(\displaystyle x=0,3\)

x=-1,3\(\displaystyle x=-1,3\)

x=1,-3\(\displaystyle x=1,-3\)

x=1,3\(\displaystyle x=1,3\)

x=2,1\(\displaystyle x=2,1\)

Correct answer:

x=1,-3\(\displaystyle x=1,-3\)

Explanation:

Factoring yields (x+3)(x-1)\(\displaystyle (x+3)(x-1)\) giving roots of -3\(\displaystyle -3\) and 1\(\displaystyle 1\).

Example Question #5 : How To Factor An Equation

\frac{-3-2x+x^{2}}{x-3}\(\displaystyle \frac{-3-2x+x^{2}}{x-3}\)

 

Find the root of the equation above.

Possible Answers:

x=0\(\displaystyle x=0\)

x=2\(\displaystyle x=2\)

x=1\(\displaystyle x=1\)

x=3\(\displaystyle x=3\)

x=-1\(\displaystyle x=-1\)

Correct answer:

x=-1\(\displaystyle x=-1\)

Explanation:

The numerator can be factored into (x-3)(x+1)\(\displaystyle (x-3)(x+1)\).

Therefore, it can cancel with the denominator. So x+1=0\(\displaystyle x+1=0\) imples x=-1\(\displaystyle x=-1\).

Example Question #6 : How To Factor An Equation

\(\displaystyle x^{2}-5x-24=0\)

 

Solve for \(\displaystyle x\).

Possible Answers:

\(\displaystyle x=8\ x=-3\)

\(\displaystyle x=-8\ x=3\)

\(\displaystyle x=\frac{5}{24}\ x=-\frac{5}{24}\)

\(\displaystyle x=\frac{24}{5}\ x=\frac{-24}{5}\)

\(\displaystyle x=24\ x=0\)

Correct answer:

\(\displaystyle x=8\ x=-3\)

Explanation:

\(\displaystyle x^{2}-5x-24=0\)

Find all factors of 24

1, 2, 3,4, 6, 8, 12, 24

Now find two factors that add up to \(\displaystyle -5\) and multiply to \(\displaystyle -24\)\(\displaystyle -8\) and \(\displaystyle 3\) are the two factors.

By factoring, you can set the equation to be \(\displaystyle (x-8)(x+3)=0\)

If you FOIL it out, it gives you \(\displaystyle x^{2}-5x-24=0\).

Set each part of the equation equal to 0, and solve for \(\displaystyle x\).

\(\displaystyle x+3=0\) and \(\displaystyle x-8=0\)

\(\displaystyle x=-3\) and \(\displaystyle x=8\)

Learning Tools by Varsity Tutors