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Grade 9

1.06 Positive and negative rational numbers

Lesson

We will now combine everything we have learned about positive and negative (directed) numbers and the order of operations with our knowledge of fractions and decimals.

Comparing and ordering rational positive and negative numbers

Greater than and less than

The symbol $<$< represents the phrase is less than. For example, $-\frac{3}{2}$32 is less than $\frac{3}{4}$34 can be represented by $-\frac{3}{2}<\frac{3}{4}$32<34.
The symbol $>$> represents the phrase is greater than. For example, $\frac{4}{3}$43 is greater than $-\frac{2}{3}$23 can be represented by $\frac{4}{3}>-\frac{2}{3}$43>23.

 

On the number line below, each tick is labelled with a multiple of the fraction $\frac{1}{5}$15. We can see that the point further to the left is plotted at the fraction $-\frac{3}{5}$35, and the point further to the right is plotted at the fraction $\frac{6}{5}$65.

 

Each tick is labelled with a fraction.

This means $\frac{6}{5}$65 is greater than $-\frac{3}{5}$35. It is the numbers' positions on the number line that helps us decide which number is greater (not their magnitudes).

 

Multiplying and dividing rational positive and negative numbers

We follow the exact same rules as before, we just need to take care when dealing with negative numbers.

Dividing rational positive and negative numbers

We can divide fractions with keep, change, flip.

  • Keep the first fraction the same
  • Change division to multiplication
  • Flip the second fraction to the reciprocal

 

Worked Example

Evaluate $-\frac{7}{10}\div\frac{4}{9}$710÷​49.

Think: Following the steps above, we want to keep the first fraction $-\frac{7}{10}$710 as is, change the division operation to multiplication, and finally take the reciprocal of (or flip) $\frac{4}{9}$49. We also know that we are dividing a negative number by a positive number, so the result will be a negative number.

Do: The reciprocal of $\frac{4}{9}$49 is $\frac{9}{4}$94. We then want to multiply by this giving us $-\frac{7}{10}\div\frac{4}{9}=-\frac{7}{10}\times\frac{9}{4}$710÷​49=710×94.

We can now multiply the numerators and denominators, $-\frac{7\times9}{10\times4}$7×910×4 and evaluate the multiplications taking care to use the right signs, $-\frac{63}{40}$6340. So $-\frac{7}{10}\div\frac{4}{9}=-\frac{63}{40}$710÷​49=6340.

Reflect: By combining all we know about rational numbers and positive and negative numbers, we can follow a series of simple steps to arrive at the correct answer. We knew before calculating anything that our answer was going to be a negative number. It is useful to know whether to expect a positive or negative answer before evaluating, as this can help us know if we have made a mistake if we end up with an answer that is not the expected sign.

 

Multiplying and dividing positive and negative decimals

As with fractions, we follow the same rules as before, taking into account if our numbers are positive and/or negative to decide whether our answer will be positive or negative.

Worked Example

Evaluate $4.83\times\left(-5.7\right)$4.83×(5.7)

Think: We are multiplying a positive number by a negative number, so we know that the product will be negative. Ignoring the signs we can now calculate $4.83\times5.7$4.83×5.7 as we normally would, remembering to add the negative sign back to our answer.

Do: Before we even begin to calculate the answer it can be a good idea to have an estimation of the answer, especially when dealing with decimals. This will help us confirm our final answer is of the right magnitude.

Rounding both numbers to the nearest whole, gives us the much simpler calculation $5\times6$5×6, which we can evaluate to get $30$30, so we would expect our answer to be close to this value, taking into account the negative, we can expect our answer to be roughly equal to $-30$30.

Now, to start the process, we ignore the decimal points. In this case we get $483$483 and $57$57. We can then multiply these together using the vertical algorithm:

      $4$4 $8$8 $3$3    
$\times$×       $5$5 $7$7    
    $3$3 $3$3 $8$8 $1$1  

Evaluating $483\times7$483×7

$+$+ $2$2 $4$4 $1$1 $5$5 $0$0  

Evaluating $483\times5\times10$483×5×10

  $2$2 $7$7 $5$5 $3$3 $1$1  

Adding the two products together

 

Now, to account for the decimal point, we add the total number of decimal places in the original numbers together.

In this case the original numbers are $4.83$4.83, which has two decimal places, and $5.7$5.7, which has one decimal place. So their product will have $2+1=3$2+1=3 decimal places.

Then to find the answer, we take the product that we calculated before and insert the decimal point such that there are $3$3 decimal places.

Finally, we need to account for the fact that we were actually multiplying by $-5.7$5.7. So our final answer will be $-27.531$27.531.

This is very close to our original estimate of $-30$30.

Summary

The reciprocal of a number is $1$1 divided by that number.

  • The reciprocal of a whole number is 1 over that number.
  • The reciprocal of a fraction can be found by swapping the numerator and denominator.

The magnitude of a number is its distance from zero.

  • For example, $6$6 and $-6$6 have the same magnitude.

Practice questions

Question 1

Which decimal is greater?

  1. $-0.74$0.74

    A

    $0.8$0.8

    B

    $-0.74$0.74

    A

    $0.8$0.8

    B

Question 2

Evaluate $-\frac{2}{5}\times\left(-\frac{9}{7}\right)$25×(97).

Question 3

Evaluate the quotient $7.36\div\left(-0.08\right)$7.36÷​(0.08)

Outcomes

9.B3.2

Apply an understanding of unit fractions and their relationship to other fractional amounts, in various contexts, including the use of measuring tools.

9.B3.3

Apply an understanding of integers to explain the effects that positive and negative signs have on the values of ratios, rates, fractions, and decimals, in various contexts.

9.B3.4

Solve problems involving operations with positive and negative fractions and mixed numbers, including problems involving formulas, measurements, and linear relations, using technology when appropriate.

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