SAT II Math I : Solving Inequalities

Study concepts, example questions & explanations for SAT II Math I

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Example Questions

Example Question #102 : Single Variable Algebra

Give the solution set of the inequality

\displaystyle \frac{2x-5}{x-7} > 0

Possible Answers:

\displaystyle \left (-\infty, 2\frac{1}{2} \right )

\displaystyle \left (-\infty, 2\frac{1}{2} \right ) \cup \left ( 7, \infty\right )

\displaystyle \left (-\infty, 2\frac{1}{2} \right ) \cup \left ( 2\frac{1}{2}, 7 \right ) \cup \left ( 7, \infty\right )

\displaystyle \left ( 2\frac{1}{2}, 7 \right )

\displaystyle \left ( 2\frac{1}{2}, 7 \right )\cup \left ( 7, \infty\right )

Correct answer:

\displaystyle \left (-\infty, 2\frac{1}{2} \right ) \cup \left ( 7, \infty\right )

Explanation:

Two numbers of like sign have a positive quotient.

Therefore, \displaystyle \frac{2x-5}{x-7} > 0 has as its solution set the set of points at which \displaystyle 2x-5 and \displaystyle x- 7 are both positive or both negative.

To find this set of points, we identify the zeroes of both expressions. 

 

\displaystyle 2x- 5 = 0

\displaystyle 2x= 5

\displaystyle x = 2\frac{1}{2}

 

\displaystyle x-7 = 0

\displaystyle x = 7

 

Since \displaystyle \frac{2x-5}{x-7} is nonzero we have to exclude \displaystyle x = 2\frac{1}{2}\displaystyle x = 7 is excluded anyway since it would bring about a denominator of zero. We choose one test point on each of the three intervals \displaystyle \left (-\infty, 2\frac{1}{2} \right ) , \left ( 2\frac{1}{2}, 7 \right ) , \left ( 7, \infty\right ) and determine where the inequality is correct.

 

\displaystyle \left (-\infty, 2\frac{1}{2} \right )

Choose \displaystyle x = 0:

\displaystyle \frac{2 \cdot 0-5}{0-7} = \frac{-5}{-7} = \frac{5}{7} > 0 - True.

 

\displaystyle \left ( 2\frac{1}{2}, 7 \right )

Choose \displaystyle x=3:

\displaystyle \frac{2 \cdot 3-5}{3-7} = \frac{6}{-4} =- \frac{3}{2} > 0 - False.

 

\displaystyle \left ( 7, \infty\right )

Choose \displaystyle x = 8:

\displaystyle \frac{2 \cdot 8-5}{8-7} = \frac{11}{1} =11> 0 - True.

 

The solution set is \displaystyle \left (-\infty, 2\frac{1}{2} \right ) \cup \left ( 7, \infty\right )

Example Question #1 : Solving Inequalities

Solve for x.

\displaystyle 2x-7\geq 1

Possible Answers:

\displaystyle x\geq -3

\displaystyle x\leq 4

\displaystyle x\geq 5

\displaystyle x> 4

\displaystyle x\geq 4

Correct answer:

\displaystyle x\geq 4

Explanation:

Solving inequalities is very similar to solving an equation. We must start by isolating x by moving the terms farthest from it to the other side of the inequality. In this case, add 7 to each side.

\displaystyle 2x-7+7\geq 1+7

\displaystyle 2x\geq 8

Now, divide both sides by 2.

\displaystyle x\geq 4

Example Question #104 : Single Variable Algebra

Solve for x.

\displaystyle 3x+2\geq -7

Possible Answers:

\displaystyle x\geq -1

\displaystyle x\geq -3

\displaystyle x\leq -3

\displaystyle x\geq 3

\displaystyle x\geq 1

Correct answer:

\displaystyle x\geq -3

Explanation:

Solving inequalities is very similar to solving an equation. We must start by isolating x by moving the terms farthest from it to the other side of the inequality. In this case, subtract 2from  each side.

\displaystyle 3x+2-2\geq -7-2

\displaystyle 3x\geq -9

Now, divide both sides by 2.

\displaystyle x\geq -3

Example Question #105 : Single Variable Algebra

Solve the following inequality: 

\displaystyle 3x+7< 9

Possible Answers:

\displaystyle x< -4

\displaystyle x< 7

\displaystyle x< 3

\displaystyle x< 9

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

Correct answer:

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

Explanation:

To solve for an inequality, you solve like you would for a single variable expression and get \displaystyle x by itself.  

First, subtract \displaystyle 7 from both sides to get,

\displaystyle 3x< 2.  

Then divide both sides by \displaystyle 3 and your final answer will be, 

\displaystyle x< \frac{2}{3}.

Example Question #101 : Single Variable Algebra

Solve the inequality:  \displaystyle 3-(3-2x)< 10

Possible Answers:

\displaystyle x< 5

\displaystyle x>-\frac{1}{5}

\displaystyle x< \frac{1}{5}

\displaystyle x< -5

\displaystyle x>5

Correct answer:

\displaystyle x< 5

Explanation:

Simplify the left side.

\displaystyle 3-3+2x< 10

The inequality becomes:

\displaystyle 2x< 10

Divide by two on both sides.

\displaystyle \frac{2}{2}x< \frac{10}{2}

The answer is:  \displaystyle x< 5

Example Question #106 : Single Variable Algebra

Solve the inequality:  \displaystyle 2x+6\leq 9x-3

Possible Answers:

\displaystyle x\geq-\frac{3}{11}

\displaystyle x\leq-\frac{3}{11}

\displaystyle x\leq-\frac{9}{7}

\displaystyle x\geq-\frac{9}{7}

\displaystyle x\geq\frac{9}{7}

Correct answer:

\displaystyle x\geq\frac{9}{7}

Explanation:

Subtract \displaystyle 2x on both sides.

\displaystyle 2x+6-2x\leq 9x-3-2x

\displaystyle 6\leq 7x-3

Add 3 on both sides.

\displaystyle 6+3\leq 7x-3+3

\displaystyle 9\leq 7x

Divide by 7 on both sides.

\displaystyle \frac{9}{7}\leq \frac{7x}{7}

\displaystyle \frac{9}{7}\leq x

The answer is:  \displaystyle x\geq\frac{9}{7}

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