NZ Level 6 (NZC) Level 1 (NCEA)
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Solve one step inequalities
Lesson

In most cases when we have an inequality involving a variable, it will be useful to simplify and solve the inequality.

We are used to solving equations, and the method for solving inequalities is very similar. Most importantly, "whatever operation is done to one side needs to be done to the other side" is still true for inequalities! 

In fact, the steps that we take are the same steps as for equations, but with one extra consideration: which way does the inequality symbol face after each step?

 

Addition and subtraction

Addition:

Numeric   Algebraic
$3$3 $<$< $4$4   $x-2$x2 $<$< $4$4
$3+2$3+2 $<$< $4+2$4+2   $x-2+2$x2+2 $<$< $4+2$4+2
$5$5 $<$< $6$6   $x$x $<$< $6$6

 

As we can see, adding a number to both sides of an inequality doesn't change the inequality symbol.

Subtraction:

Numeric   Algebraic
$5$5 $>$> $2$2   $x+1$x+1 $\ge$ $2$2
$5-1$51 $>$> $2-1$21   $x+1-1$x+11 $\ge$ $2-1$21
$4$4 $>$> $1$1   $x$x $\ge$ $1$1

 

Similarly, subtracting a number from both sides of an inequality doesn't change the inequality symbol.

 

Multiplication and division

Multiplication by positive numbers:

Numeric   Algebraic
$4$4 $<$< $7$7   $\frac{x}{3}$x3 $\le$ $7$7
$4\times3$4×3 $<$< $7\times3$7×3   $\frac{x}{3}\times3$x3×3 $\le$ $7\times3$7×3
$12$12 $<$< $21$21   $x$x $\le$ $21$21

 

Multiplication by negative numbers:

Numeric   Algebraic
$4$4 $<$< $7$7   $-\frac{x}{3}$x3 $<$< $7$7
$4\times\left(-3\right)$4×(3) $>$> $7\times\left(-3\right)$7×(3)   $\left(-\frac{x}{3}\right)\times\left(-3\right)$(x3)×(3) $>$> $7\times\left(-3\right)$7×(3)
$-12$12 $>$> $-21$21   $x$x $>$> $-21$21

 

Now we have found a difference! Multiplying both sides of an inequality by a positive number doesn't change the inequality symbol, but multiplying by a negative number does change the inequality symbol.

Division:

Remember that we can think about division as an equivalent multiplication. For example, dividing by $3$3 is the same as multiplying by $\frac{1}{3}$13. So we get the same result: dividing both sides of an inequality by a positive number doesn't change the inequality symbol, but dividing by a negative number does change the inequality symbol.

 

Writing an inequality in reverse order

It is often useful to write an equation in reverse order. For example, if we reach the step $3=x$3=x in our working, we often reverse this and write $x=3$x=3

We can also do this with inequalities, as long as we reverse the symbol as well. For example, "$3$3 is greater than $x$x" means the same thing as "$x$x is less than $3$3". So $3>x$3>x can be rewritten as $x<3$x<3, using the opposite symbol.

 

Summary

Same symbol

The following operations don't change the inequality symbol used: 

  • Adding a number to both sides of an inequality.
  • Subtracting a number from both sides of an inequality.
  • Multiplying both sides of an inequality by a positive number.
  • Dividing both sides of an inequality by a positive number.
Opposite symbol

The following operations reverse the inequality symbol used: 

  • Writing an inequality in reverse order.
  • Multiplying both sides of an inequality by a negative number.
  • Dividing both sides of an inequality by a negative number.

 

Practice questions

QUESTION 1

Solve the following inequality: $x+5>14$x+5>14

Question 2

Solve the following inequality: $x+5\ge11$x+511

Question 3

Solve the following inequality: $10x<90$10x<90

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

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