GMAT Math : Absolute Value

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #433 : Algebra

Solve \left | 3x - 7 \right |=8\(\displaystyle \left | 3x - 7 \right |=8\).

Possible Answers:

\(\displaystyle x=5\)

\(\displaystyle x=5\) or \(\displaystyle x=\)\(\displaystyle \dpi{100} -\frac{1}{3}\)

\(\displaystyle x=5\) or \(\displaystyle x=\) \(\displaystyle \frac{1}{3}\)

\(\displaystyle x=-\frac{1}{3}\)

\(\displaystyle x=-5\) or \(\displaystyle x=\) \(\displaystyle \frac{1}{3}\)

Correct answer:

\(\displaystyle x=5\) or \(\displaystyle x=\)\(\displaystyle \dpi{100} -\frac{1}{3}\)

Explanation:

\left | 3x - 7 \right |=8\(\displaystyle \left | 3x - 7 \right |=8\) really consists of two equations: 3x - 7 = \pm 8\(\displaystyle 3x - 7 = \pm 8\)

We must solve them both to find two possible solutions.

3x - 7 = 8 \Rightarrow 3x = 15\Rightarrow x = 5\(\displaystyle 3x - 7 = 8 \Rightarrow 3x = 15\Rightarrow x = 5\)

3x - 7 = - 8 \Rightarrow 3x = -1\Rightarrow x = -1/3\(\displaystyle 3x - 7 = - 8 \Rightarrow 3x = -1\Rightarrow x = -1/3\)

So \(\displaystyle x=5\) or \(\displaystyle x=\) \(\displaystyle \dpi{100} -\frac{1}{3}\).

Example Question #1 : Absolute Value

Solve \left | 2x - 5 \right |\geq 3\(\displaystyle \left | 2x - 5 \right |\geq 3\).

Possible Answers:

1 < x < 4\(\displaystyle 1 < x < 4\)

x < 1, x > 4\(\displaystyle x < 1, x > 4\)

-2 \leq x\leq 5\(\displaystyle -2 \leq x\leq 5\)

x \leq -1, x\geq -4\(\displaystyle x \leq -1, x\geq -4\)

x \leq 1, x\geq 4\(\displaystyle x \leq 1, x\geq 4\)

Correct answer:

x \leq 1, x\geq 4\(\displaystyle x \leq 1, x\geq 4\)

Explanation:

It's actually easier to solve for the complement first.  Let's solve \left | 2x-5 \right |<3\(\displaystyle \left | 2x-5 \right |< 3\).  That gives -3 < 2x - 5 < 3.  Add 5 to get 2 < 2x < 8, and divide by 2 to get 1 < x < 4.  To find the real solution then, we take the opposites of the two inequality signs.  Then our answer becomes x\leq 1 \textsc{ or } x\geq 4\(\displaystyle x\leq 1 \textsc{ or } x\geq 4\).

Example Question #3 : Understanding Absolute Value

Give the \(\displaystyle x\)-intercept(s), if any, of the graph of the function \(\displaystyle f (x) = \left | 4x + B \right | - 8\) in terms of \(\displaystyle B\)

Possible Answers:

\(\displaystyle \left ( -B-2, 0 \right ), \left ( -B+2, 0 \right )\)

\(\displaystyle (B- 8, 0)\)

\(\displaystyle \left ( \frac{-B -8 }{4}, 0 \right ), \left ( \frac{-B +8 }{4}, 0 \right )\)

\(\displaystyle \left ( B-2, 0 \right ), \left ( B+2, 0 \right )\)

\(\displaystyle \left ( \frac{B -8 }{4}, 0 \right ), \left ( \frac{B +8 }{4}, 0 \right )\)

Correct answer:

\(\displaystyle \left ( \frac{-B -8 }{4}, 0 \right ), \left ( \frac{-B +8 }{4}, 0 \right )\)

Explanation:

Set \(\displaystyle f (x) = 0\) and solve for \(\displaystyle x\):

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

\(\displaystyle \left | 4x + B \right | - 8 = 0\)

\(\displaystyle \left | 4x + B \right | = 8\)

 

Rewrite as a compound equation and solve each part separately:

\(\displaystyle 4x + B = -8 \textrm{ or } 4x + B = 8\)

 

\(\displaystyle 4x + B = -8\)

\(\displaystyle 4x + B -B = -B -8\)

\(\displaystyle 4x = -B -8\)

\(\displaystyle 4x \div 4 =\left ( -B -8 \right ) \div 4\)

\(\displaystyle x = \frac{-B -8 }{4}\)

 

 

\(\displaystyle 4x + B = 8\)

\(\displaystyle 4x + B -B = -B + 8\)

\(\displaystyle 4x = -B +8\)

\(\displaystyle 4x \div 4 =\left ( -B +8 \right ) \div 4\)

\(\displaystyle x = \frac{-B +8 }{4}\)

Example Question #436 : Algebra

A number is ten less than its own absolute value. What is this number?

Possible Answers:

\(\displaystyle 5\)

\(\displaystyle 10\)

No such number exists.

\(\displaystyle -5\)

\(\displaystyle -10\)

Correct answer:

\(\displaystyle -5\)

Explanation:

We can rewrite this as an equation, where \(\displaystyle N\) is the number in question:

\(\displaystyle N = |N| - 10\)

A nonnegative number is equal to its own absolute value, so if this number exists, it must be negative.

In thsi case, \(\displaystyle |N| = - N\), and we can rewrite that equation as

\(\displaystyle N = -N- 10\)

\(\displaystyle N +N= -N- 10 +N\)

\(\displaystyle 2N = -10\)

\(\displaystyle 2N \div 2 = -10\div 2\)

\(\displaystyle N = -5\)

This is the only number that fits the criterion.

Example Question #4 : Understanding Absolute Value

If \(\displaystyle -1< n< 0\), which of the following has the greatest absolute value?

Possible Answers:

\(\displaystyle \frac{n}{2}\)

\(\displaystyle \frac{1}{n^{2}}\)

\(\displaystyle n+1\)

\(\displaystyle n^2\)

\(\displaystyle \frac{1}{n}\)

Correct answer:

\(\displaystyle \frac{1}{n^{2}}\)

Explanation:

Since \(\displaystyle -1< n< 0\), we know the following:  

\(\displaystyle 0< n^2< 1\) ;

\(\displaystyle -1< \frac{n}{2}< 0\);

\(\displaystyle \frac{1}{n^2}>1\);

\(\displaystyle \frac{1}{n}< -1\);

\(\displaystyle 0< n+1< 1\).

Also, we need to compare absolute values, so the greatest one must be either \(\displaystyle \left | \frac{1}{n^2}\right |\) or \(\displaystyle \left | \frac{1}{n}\right |\).

We also know that \(\displaystyle \left | n^2\right |< \left | n\right |\) when \(\displaystyle -1< n< 0\).

Thus, we know for sure that \(\displaystyle \left | \frac{1}{n^2}\right |>\left | \frac{1}{n}\right |\).

 

Example Question #5 : Understanding Absolute Value

Give all numbers that are twenty less than twice their own absolute value.

Possible Answers:

No such number exists.

\(\displaystyle - 10 \textrm{ and } 10\)

\(\displaystyle - 6\frac{2}{3} \textrm{ and } 20\)

\(\displaystyle - 6\frac{2}{3} \textrm{ and } 10\)

\(\displaystyle - 10 \textrm{ and } 20\)

Correct answer:

\(\displaystyle - 6\frac{2}{3} \textrm{ and } 20\)

Explanation:

We can rewrite this as an equation, where \(\displaystyle N\) is the number in question:

\(\displaystyle N = 2\cdot |N| - 20\)

If \(\displaystyle N\) is nonnegative, then \(\displaystyle | N | = N\), and we can rewrite this as 

\(\displaystyle N = 2N - 20\)

Solve:

\(\displaystyle N -2N= 2N - 20-2N\)

\(\displaystyle -N= - 20\)

\(\displaystyle N = 20\)

 

If \(\displaystyle N\) is negative, then \(\displaystyle | N | =- N\), and we can rewrite this as 

\(\displaystyle N = 2\left (-N \right ) - 20\)

\(\displaystyle N = -2N - 20\)

\(\displaystyle N + 2N= -2N - 20 + 2N\)

\(\displaystyle 3N=- 20\)

\(\displaystyle 3N \div 3=- 20\div 3\)

\(\displaystyle N=- 6\frac{2}{3}\)

 

The numbers \(\displaystyle - 6\frac{2}{3}, 20\) have the given characteristics.

Example Question #7 : Absolute Value

Solve for \(\displaystyle x\) in the absolute value equation

 

\(\displaystyle 1-\frac{5}{2}\left |x-5 \right | = 15\)

Possible Answers:

\(\displaystyle -\frac{3}{5}\)

None of the other answers

\(\displaystyle -\frac{53}{3}\)

\(\displaystyle \frac{53}{3}\)

\(\displaystyle \frac{3}{5}\)

Correct answer:

None of the other answers

Explanation:

The correct answer is that there is no \(\displaystyle x\).

 

We start by adding \(\displaystyle 1\) to both sides giving

 

\(\displaystyle -\frac{5}{2} \left|x-5\right|=14\)

 

Then multiply both sides by \(\displaystyle 2\).

 

\(\displaystyle -5\left|x-5|=28\)

 

Then divide both sides by \(\displaystyle -5\)

 

\(\displaystyle |x-5|=-\frac{28}{5}\)

 

Now it is impossible to go any further. The absolute value of any quantity is always positive (or sometimes \(\displaystyle 0\)). Here we have the absolute value of something equaling a negative number. That's never possible, hence there is no \(\displaystyle x\) that makes this a true equation.

Example Question #8 : Absolute Value

Solve the following equation:

 \(\displaystyle 32-\left | x-23\right |=13\)

Possible Answers:

\(\displaystyle x={10,19}\)

\(\displaystyle x={-22,10}\)

\(\displaystyle x={-19,19}\)

\(\displaystyle x={-23,0}\)

\(\displaystyle x={42,4}\)

Correct answer:

\(\displaystyle x={42,4}\)

Explanation:

We start by isolating the expression with the absolute value:

\(\displaystyle 32-\left | x-23\right |=13\)  becomes \(\displaystyle -\left | x-23\right |=13-32=-19\)

So: \(\displaystyle x-23=19\) or \(\displaystyle x-23=-19\)

We then solve the two equations above, which gives us 42 and 4 respectively.

So the solution is \(\displaystyle x={42,4}\)

Example Question #3 : Understanding Absolute Value

Solve the absolute value equation for \(\displaystyle x\).

 

\(\displaystyle |2x-5|+3=5\)

Possible Answers:

The equation has no solution

\(\displaystyle x = 2,\frac{3}{2}\)

None of the other answers.

\(\displaystyle x = -\frac{7}{2}, \frac{7}{2}\)

\(\displaystyle x = \frac{7}{2}, \frac{3}{2}\)

Correct answer:

\(\displaystyle x = \frac{7}{2}, \frac{3}{2}\)

Explanation:

We proceed as follows

 

\(\displaystyle |2x-5| +3 =5\) (Start)

\(\displaystyle |2x -5| =2\) (Subtract 3 from both sides)

\(\displaystyle 2x-5 =2\) or \(\displaystyle 2x-5 = -2\) (Quantity inside the absolute value can be positive or negative)

 

\(\displaystyle 2x = 7\) or \(\displaystyle 2x = 3\) (add five to both sides)

\(\displaystyle x = \frac{7}{2}\) or \(\displaystyle x = \frac{3}{2}\)

 

Another way to say this is \(\displaystyle x = \frac{7}{2},\frac{3}{2}\)

Example Question #6 : Understanding Absolute Value

\(\displaystyle \left | 2m-\frac{1}{2}\right |< 4\)

Which of the following could be a value of \(\displaystyle m\)?

 

Possible Answers:

\(\displaystyle -\frac{7}{2}\)

\(\displaystyle \frac{7}{4}\)

\(\displaystyle -\frac{7}{4}\)

\(\displaystyle \frac{9}{2}\)

\(\displaystyle \frac{9}{4}\)

Correct answer:

\(\displaystyle \frac{7}{4}\)

Explanation:

To solve an inequality we need to remember what the absolute value sign says about our expression. In this case it says that

     \(\displaystyle \left | 2m-\frac{1}{2}\right |< 4\)

can be written as

\(\displaystyle 2m-\frac{1}{2}< 4\) Of \(\displaystyle 2m-\frac{1}{2}>-4\).

Rewriting this in one inequality we get:

\(\displaystyle -4< 2m-\frac{1}{2}< 4\)

From here we add one half to both sides .

\(\displaystyle -\frac{7}{2}< 2m< \frac{9}{2}\)

Finally, we divide by two to isolate and solve for m.

\(\displaystyle -\frac{7}{4}< m< \frac{9}{4}\)

\(\displaystyle -1.75< m< 2.25\)

Only \(\displaystyle \frac{7}{4}\) is between -1.75 and 2.25

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