NZ Level 6 (NZC) Level 1 (NCEA)
topic badge
Solve two step inequalities
Lesson

We have previously looked at solving simple inequalities, using just one operation at a time. We learned that the process is almost identical to that of solving equations, but we also need to keep in mind which operations cause the inequality symbol to reverse.

In particular, we found that multiplying or dividing by a negative number causes the inequality symbol to change direction. Also, writing an inequality in reverse order causes the inequality symbol to reverse.

 

Exploration

Let's take a look at solving a slightly more complicated inequality, such as $-3x+2\ge14$3x+214. There are now two operations being applied to $x$x (multiplication and addition). Much like solving equations with two (or more) operations, we will need to take the order of operations into consideration as well.

Looking at the inequality $-3x+2\ge14$3x+214 and thinking about the order of operations, we can see that $x$x is first multiplied by $-3$3 and then $2$2 is added. To solve this inequality, we want to undo these operations in reverse order. That is, we can solve this inequality by first subtracting $2$2 from both sides, then dividing both sides by $-3$3 (which will change the inequality symbol used):

$-3x+2$3x+2 $\ge$ $14$14    
$-3x+2-2$3x+22 $\ge$ $14-2$142   Subtracting $2$2 from both sides
$-3x$3x $\ge$ $12$12   Simplifying
$\frac{-3x}{-3}$3x3 $\le$ $\frac{12}{-3}$123   Dividing both sides by $-3$3 (and changing the inequality symbol)
$x$x $\le$ $-4$4   Simplifying

In this case, we arrive at the result $x\le-4$x4. We can test some values in the original inequality to see if this is the right solution set - let's say $x=-5$x=5 and $x=-3$x=3.

  • When $x=-5$x=5, we have $-3x+2=-3\times\left(-5\right)+2=17$3x+2=3×(5)+2=17, which is greater than or equal to $14$14.
  • When $x=-3$x=3, we have $-3x+2=-3\times\left(-3\right)+2=11$3x+2=3×(3)+2=11, which is not greater than or equal to $14$14.

So our result of $x\le-4$x4 seems to be correct.

 

Summary

When solving any inequality:

  • Multiplying or dividing both sides of an inequality by a negative number reverses the inequality symbol.
  • Writing an inequality in reverse order also reverses the inequality symbol.

When solving an inequality with two (or more) operations:

  • It is generally easiest to undo one operation at a time, in reverse order to the order of operations.

 

Practice questions

Question 1

Solve the following inequality: $3x+27>3$3x+27>3

Question 2

Solve the following inequality: $6x-54\ge30$6x5430

Question 3

Solve the following inequality: $\frac{a}{5}+3>3$a5+3>3

Outcomes

NA6-5

Form and solve linear equations and inequations, quadratic and simple exponential equations, and simultaneous equations with two unknowns

91027

Apply algebraic procedures in solving problems

What is Mathspace

About Mathspace