GMAT Math : Algebra

Study concepts, example questions & explanations for GMAT Math

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Example Questions

Example Question #11 : Absolute Value

Define a function \displaystyle f to be

\displaystyle f(x)= |3x- 12 | + 5

Give the range of the function.

Possible Answers:

\displaystyle [-5, \infty)

\displaystyle [-4, \infty)

\displaystyle [4, \infty)

\displaystyle [5, \infty)

\displaystyle (-\infty, \infty)

Correct answer:

\displaystyle [5, \infty)

Explanation:

An absolute value of a number must always assume a nonnegative value, so

\displaystyle |3x- 12 | \ge 0, and

\displaystyle |3x- 12 | + 5\ge 0 +5

Therefore, 

\displaystyle f(x)=|3x- 12 | + 5\ge 5

and the range of \displaystyle f is the set \displaystyle [5, \infty).

Example Question #12 : Absolute Value

Solve the following equation:

\displaystyle \left | -4+3(-5)-(-6)\right |+3x=\left |7-2(4) \right |

Possible Answers:

\displaystyle x=-4

\displaystyle x=\frac{14}{3}

\displaystyle x=8

\displaystyle x=4

\displaystyle x=-\frac{14}{3}

Correct answer:

\displaystyle x=-4

Explanation:

Before we apply the absolute value to the two terms in the equation, we simplify what's inside of them first:

\displaystyle \left | -4+3(-5)-(-6)\right |+3x=\left |7-2(4) \right |

\displaystyle \left | -4-15+6\right |+3x=\left |7-8 \right |

\displaystyle \left | -13\right |+3x=\left |-1 \right |

Now we can apply the absolute value to each term. Remember that taking the absolute value of a quantity results in solely its value, regardless of what its sign was before the absolute value was taken. This means that that absolute value of a number is always positive:

\displaystyle \left | -13\right |+3x=\left |-1 \right |

\displaystyle 13+3x=1

\displaystyle 3x=-12

\displaystyle x=-4

Example Question #12 : Absolute Value

Give the range of the function

\displaystyle f(x)= |x-10|+ |x-6|

Possible Answers:

\displaystyle [-4, \infty)

\displaystyle [10, \infty)

\displaystyle [-6, \infty)

\displaystyle [4, \infty)

\displaystyle [6, \infty)

Correct answer:

\displaystyle [4, \infty)

Explanation:

The key to answering this question is to note that this equation can be rewritten in piecewise fashion. 

If \displaystyle x \ge 10, since both \displaystyle x-10 and \displaystyle x-6 are nonnegative, we can rewrite \displaystyle f as

\displaystyle f(x)= x-10 + x-6, or

\displaystyle f(x) = 2x-16.

On  \displaystyle [10, \infty), this has as its graph a line with positive slope, so it is an increasing function. The range of this part of the function is \displaystyle [f(10), \infty), or, since

\displaystyle f(10)= |10-10|+|10-6| = 0 + 4 = 4

\displaystyle [4, \infty).

 

If \displaystyle 6 < x < 10, since \displaystyle x-10 is negative and \displaystyle x-6 is positive, we can rewrite \displaystyle f as

\displaystyle f(x)= -x+10 + x-6, or

\displaystyle f(x)=4

\displaystyle f is a constant function on this interval and its range is \displaystyle \{4 \}.

 

If \displaystyle x \le 6, since both \displaystyle x-10 and \displaystyle x-6 are nonpositive, we can rewrite \displaystyle f as

\displaystyle f(x)= -x+10 -x+6, or

\displaystyle f(x) = -2x+16.

On  \displaystyle [- \infty, 6), this has as its graph a line with negative slope, so it is a decreasing function. The range of this part of the function is \displaystyle [f(6), \infty), or, since 

\displaystyle f(6)= |6-10|+|6-6| = 4+0=4\displaystyle [4, \infty).

 

The union of the ranges is the range of the function - \displaystyle [4, \infty).

Example Question #11 : Understanding Absolute Value

Give the range of the function

\displaystyle f(x)= |x-10|- |x-6|

Possible Answers:

None of the other choices gives a correct answer.

\displaystyle [-10,-6]

\displaystyle [-10,6]

\displaystyle [-6,10]

\displaystyle [6,10]

Correct answer:

None of the other choices gives a correct answer.

Explanation:

The key to answering this question is to note that this equation can be rewritten in piecewise fashion. 

 

If \displaystyle x > 10, since both \displaystyle x-10 and \displaystyle x-6 are positive, we can rewrite \displaystyle f as

\displaystyle f(x)= x-10 - x+6, or

\displaystyle f(x)= -4,

a constant function with range \displaystyle \left \{ -4\right \}.

 

If \displaystyle 6 \le x \le 10, since \displaystyle x-10 is negative and \displaystyle x-6 is positive, we can rewrite \displaystyle f as

\displaystyle f(x)=-x+10-x+6, or

\displaystyle f(x)=-2x+16

This is decreasing, as its graph is a line with negative slope. The range is \displaystyle [f(10), f(6)],

or, since

\displaystyle f(10)= |10-10|- |10-6| = 0 - 4 = -4

and

\displaystyle f(6)= |6-10|- |6-6| = 4 - 0 = 4,

\displaystyle [-4,4].

 

If \displaystyle x < 6, since both \displaystyle x-10 and \displaystyle x-6 are negative, we can rewrite \displaystyle f as

\displaystyle f(x)= -x+10 +x-6, or

\displaystyle f(x)= 4,

a constant function with range \displaystyle \left \{ 4\right \}.

 

The union of the ranges is the range of the function - \displaystyle [-4,4] - which is not among the choices.

Example Question #21 : Absolute Value

Simplify the following expression:

\displaystyle \tiny \tiny \small \left | -34\right |*\left | -2\right |*-\left | -0.5\right |

Possible Answers:

\displaystyle \small 170

\displaystyle \small -2

\displaystyle \small -34

\displaystyle \small 34

\displaystyle \small \small -2

Correct answer:

\displaystyle \small -34

Explanation:

This question plays a few tricks dealing with absolute values. To begin, we can recognize that any negative sign within an absolute value can basically be rendered positive. So this:

\displaystyle \tiny \tiny \small \left | -34\right |*\left | -2\right |*-\left | -0.5\right |

becomes:

\displaystyle \small 34*2*-\left | 0.5\right |

In this case, we still have a negative that was outside of the absolute value sign. This term will stay negative, so we get:

\displaystyle \small \small 34*2*- 0.5=-34 

This makes our answer \displaystyle \small -34.

 

Example Question #1531 : Problem Solving Questions

Solve the following inequality:

\displaystyle \left | 2x+3\right |< 6

Possible Answers:

\displaystyle -2< x< 6

\displaystyle -\frac{3}{2}< x< \frac{9}{2}

\displaystyle -6< x< 2

\displaystyle 3< x< 6

\displaystyle -\frac{9}{2}< x< \frac{3}{2}

Correct answer:

\displaystyle -\frac{9}{2}< x< \frac{3}{2}

Explanation:

To solve this absolute value inequality, we must remember that the absolute value of a function that is less than a certain number must be greater than the negative of that number. Using this knowledge, we write the inequality as follows, and then perform some algebra to solve for \displaystyle x:

\displaystyle \left | 2x+3\right |< 6

\displaystyle -6< 2x+3< 6

\displaystyle -9< 2x< 3

\displaystyle -\frac{9}{2}< x< \frac{3}{2}

Example Question #22 : Understanding Absolute Value

Solve the following inequality:

\displaystyle \left | 2x-3\right |>5

Possible Answers:

\displaystyle x< -1   or   \displaystyle x>4 

\displaystyle x< -4   or   \displaystyle x>1

\displaystyle x< -2   or   \displaystyle x>5

\displaystyle -4< x< 1

\displaystyle -1< x< 4

Correct answer:

\displaystyle x< -1   or   \displaystyle x>4 

Explanation:

To solve this absolute value inequality, we must remember that the absolute value of a function that is greater than a certain number is also less than the negative of that number. With this in mind, we rewrite the inequality as follows and then solve for the possible intervals of \displaystyle x:

\displaystyle \left | 2x-3\right |>5

\displaystyle 2x-3< -5   or   \displaystyle 2x-3>5

\displaystyle 2x< -2   or   \displaystyle 2x>8

\displaystyle x< -1   or   \displaystyle x>4

Example Question #452 : Algebra

\displaystyle \left | -4\right |-\left | -11\right |+18-\left | -23+2\right |=

Possible Answers:

\displaystyle 4

\displaystyle -10

\displaystyle 0

\displaystyle -18

\displaystyle 46

Correct answer:

\displaystyle -10

Explanation:

Remember that the absolute value of any number is its positive value, regardless of whether or not the number is negative before the absolute value is taken. We start by simplifying any expressions inside the absolute value signs:

\displaystyle \left | -4\right |-\left | -11\right |+18-\left | -23+2\right |

\displaystyle \left | -4\right |-\left | -11\right |+18-\left | -21\right |

Now we apply the absolute values and solve the expression:

\displaystyle \left | -4\right |-\left | -11\right |+18-\left | -21\right |

\displaystyle 4-11+18-21=-10

Example Question #21 : Absolute Value

Solve for \displaystyle x

\displaystyle \left | 6x-4\right |=14

Possible Answers:

Not enough information to solve 

 \displaystyle x=-3 and \displaystyle x=\frac{5}{3}

 \displaystyle x=3

 \displaystyle x=3 and  \displaystyle x=-\frac{5}{3}

 \displaystyle x=-\frac{5}{3}

Correct answer:

 \displaystyle x=3 and  \displaystyle x=-\frac{5}{3}

Explanation:

In order to solve the given absolute value equation, we need to solve for \displaystyle x for the two ways in which this absolute value can be solved:

1.) \displaystyle 6x-4=14

2.) \displaystyle 6x-4=-14

Solving Equation 1:

\displaystyle 6x-4=14

\displaystyle 6x=18

\displaystyle x=3

Solving Equation 2:

\displaystyle 6x-4=-14

\displaystyle 6x=-10

\displaystyle x=-\frac{10}{6}=-\frac{5}{3}

Therefore, there are two solutions to the absolute value equation: \displaystyle x=3 and \displaystyle x=-\frac{5}{3}

Example Question #23 : Understanding Absolute Value

Solve for \displaystyle z

\displaystyle \left | 10z+12\right |=112

Possible Answers:

 \displaystyle z=-10 and \displaystyle z=\frac{62}{5}

\displaystyle z=-\frac{62}{5}

 \displaystyle z=10

Not enough information to solve

 \displaystyle z=10 and \displaystyle z=-\frac{62}{5}

Correct answer:

 \displaystyle z=10 and \displaystyle z=-\frac{62}{5}

Explanation:

In order to solve the given absolute value equation, we need to solve for \displaystyle z in the two ways in which this absolute value can be solved:

1.)\displaystyle 10z+12=112

2.) \displaystyle 10z+12=-112

Solving Equation 1:

\displaystyle 10z+12=112

\displaystyle 10z=100

\displaystyle z=10

Solving Equation 2:

\displaystyle 10z+12=-112

\displaystyle 10z=-124

\displaystyle z=-\frac{124}{10}=-\frac{62}{5}

Therefore, there are two correct values of \displaystyle z\displaystyle z=10 and \displaystyle z=-\frac{62}{5}.

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