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SAT Math : How to find excluded values

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Example Question #1 : How To Find Excluded Values

Which of the following are answers to the equation below?

$\frac{x^2-4}{x^2+5x+6}=0$

I. -3

II. -2

III. 2

Possible Answers:

II and III

I, II, and III

II only

I only

III only

Correct answer:

III only

Explanation:

Given a fractional algebraic equation with variables in the numerator and denominator of one side and the other side equal to zero, we rely on a simple concept. Zero divided by anything equals zero. That means we can focus in on what values make the numerator (the top part of the fraction) zero, or in other words,

x^2-4=0

The expression x^2-4 is a difference of squares that can be factored as

(x-2)(x+2)=0

Solving this for gives either or . That means either of these values will make our numerator equal zero. We might be tempted to conclude that both are valid answers. However, our statement earlier that zero divided by anything is zero has one caveat. We can never divide by zero itself. That means that any values that make our denominator zero must be rejected. Therefore we must also look at the denominator.

x^2+5x+6=0

The left side factors as follows

(x+3)(x+2)=0

This means that if is or , we end up dividing by zero. That means that cannot be a valid solution, leaving as the only valid answer. Therefore only #3 is correct.

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Example Question #1 : How To Find Excluded Values

Which of the following provides the complete solution set for |2-z|>0 ?

Possible Answers:

$\dpi{100} z|z\neq 2$

z=2

z<2

No solutions

z>2

Correct answer:

$\dpi{100} z|z\neq 2$

Explanation:

The absolute value will always be positive or 0, therefore all values of z will create a true statement as long as $2-z\neq 0$ . Thus all values except for 2 will work.

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Example Question #2 : How To Find Excluded Values

If the average (arithmetic mean) of , , and is , what is the average of x+2 , y-6 , and ?

Possible Answers:

There is not enough information to determine the answer.

Correct answer:

Explanation:

If we can find the sum of $\dpi{100} \small x+2$ , $\dpi{100} \small y-6$ , and 10, we can determine their average. There is not enough information to solve for $\dpi{100} \small x$ or $\dpi{100} \small y$ individually, but we can find their sum, $\dpi{100} \small x+y$ .

Write out the average formula for the original three quantities. Remember, adding together and dividing by the number of quantities gives the average: $\frac{x + y + 9}{3} = 12$

Isolate $\dpi{100} \small x+y$ :

$x + y + 9 = 36$

$x + y = 27$

Write out the average formula for the new three quantities:

$\frac{x + 2 + y - 6 + 10}{3} = ?$

Combine the integers in the numerator:

$\frac{x + y + 6}{3} = ?$

Replace $\dpi{100} \small x+y$ with 27:

$\frac{27+ 6}{3} = \frac{33}{3} = 11$

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Example Question #3 : How To Find Excluded Values

Find the excluded values of the following algebraic fraction

$\frac{2x^2-5x-12}{x^2-11x+28}$

Possible Answers:

The numerator cancels all the binomials in the denomniator so ther are no excluded values.

$x\neq-11$

$x\neq4,7$

$x\neq7$

$x\neq4$

Correct answer:

$x\neq4,7$

Explanation:

To find the excluded values of a algebraic fraction you need to find when the denominator is zero. To find when the denominator is zero you need to factor it. This denominator factors into

(x-7)(x-4)

so this is zero when x=4,7 so our answer is

$x\neq4,7$

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Example Question #2 : How To Find Excluded Values

$y=\frac{x^2-5}{x^2-25}$

For what value(s) of x is the function undefined?

Possible Answers:

x=5, x=-5, x=0

x=-5

$x=-5, x=\sqrt{5}$

$x=\sqrt{5}$

x=5, x=-5

Correct answer:

x=5, x=-5

Explanation:

When the denominator of a function is equal to 0, the function is undefined at that point. We can set x²-25 equal to 0 in order to find out what values of x make that true.

x^2-25=0

We can factor to solve for x.

(x+5)(x-5)=0

x+5=0, x-5=0

x=5, x=-5

$x=\sqrt{5}$ is an incorrect answer because for this value of x, the function equals zero, but it is not undefined.

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Example Question #3 : How To Find Excluded Values

Find the extraneous solution for

$\sqrt{x}=3x-4$

Possible Answers:

$x=\frac{16}{9}$

x=1

x=-1

There are no extraneous solutions

Correct answer:

x=1

Explanation:

$\sqrt{x}=3x-4$

$(\sqrt{x})^2=(3x-4)^2$

x=9x^2-24x+16

0=9x^2-25x+16

$x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$

$x=\frac{25\pm\sqrt{(-25)^2-4\cdot9\cdot16}}{2\cdot9}$

$x=\frac{25\pm\sqrt{625-576}}{18}$

$x=\frac{25\pm\sqrt{49}}{18}$

$x=\frac{25+7}{18}=\frac{32}{18}=\frac{16}{9}$

$x=\frac{25-7}{18}=\frac{18}{18}=1$

Now lets plug these values into our original equation.

For $x=\frac{16}{9}$

$\sqrt{x}=3x-4$

$\sqrt{\frac{16}{9}}=3\left(\frac{16}{9}\right)-4$

$\frac{4}{3}=\frac{16}{3}-4$

$\frac{4}{3}=\frac{16}{3}-\frac{12}{3}=\frac{4}{3}$

So this isn't an extraneous solution.

For x=1

$\sqrt{x}=3x-4$

$\sqrt{1}=3(1)-4$

1=3-4=-1

Since $1\neq-1$ , x=1 is an extraneous solution.

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SAT Math : How to find excluded values

Study concepts, example questions & explanations for SAT Math

All SAT Math Resources

Example Questions

Example Question #1 : How To Find Excluded Values

Example Question #1 : How To Find Excluded Values

Example Question #2 : How To Find Excluded Values

Example Question #3 : How To Find Excluded Values

Example Question #2 : How To Find Excluded Values

Example Question #3 : How To Find Excluded Values

All SAT Math Resources

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