6.4.1 Dividing Fractions with the Calculator
In this exploration, you will learn how to use your calculator to divide fractions, and also discover a useful method for dividing fractions by hand.
The calculator can be accessed on the left-hand side bar under the Toolkit.
Calculator Exploration: Dividing Fractions
For this activity, suppose that you have three pizzas, and you divide each of them in half.
How many halves do you have? You can see that there are six halves in three pizzas. When you calculate , you can think of this as “How many halves are there in three?”
Try the calculation on the calculator. Remember to put in parentheses, because you are dividing three by the entire fraction . Do you get the answer six?
The key sequence is
Remember that a division sign is equivalent to a fraction bar, and this is how the calculator displays the calculation. The fraction bar under the three is larger than the one in the middle of the , indicating that you are dividing 3 by the fraction .
To show that the parentheses are not optional, enter the same calculation without parentheses.
The calculator follows the order of operations. There are two divisions, so the calculator works from the left and divides three by one, then the result by two.
Can you see how the length of the fraction bars shows this? Compare this result to your previous expression, where you put the into parentheses. Whenever you divide fractions, take great care to use parentheses around each fraction so that the calculator does as you intend.
Calculator Exploration: Practice with Dividing Fractions
Now try the following divisions. First, think about what the answer should be, and then try the calculation on the calculator.
Remember to clear the last calculation before you start, and don’t forget to put the fraction in parentheses.
Can you see a pattern in the calculations? Look at them again and see if you can describe what’s happening.
Look at the last three calculations. They asked you to divide 3 by , by , and 3 by .
How do you create the result from the numbers involved?
When you divide 3 by , you get 6, which is the same as multiplying 3 by 2.
When you divide 3 by , you get 9, which is the same as multiplying 3 by 3.
When you divide 3 by , you get 12, which is the same as multiplying 3 by 4.
Calculator Exploration: Further Investigation
Let’s investigate further. Try the following calculations, and make a note of the answers.
In each case, you should obtain the same answer for the division as for the multiplication. For (a), you should get or for (b) or ; and for (c) the answer is 7.
Look again for a pattern. In each case, you start with 3. Look at the fraction that you divide by, and the fraction that you multiply by. What is the connection in each case?
In each case, the fraction that you multiply by is the fraction that you divide by turned upside down. In other words, the numerator and the denominator switch places.
This is the exciting part of mathematics, spotting patterns and then seeing whether they hold for other cases. We have a conjecture!
If you think you have spotted the pattern, then let's try to write it out. Completing the following sentence:
To divide by a fraction …
To divide by a fraction, switch the numerator and the denominator, then multiply.
There is another way to write this. When the numerator and the denominator trade places, the result is called the reciprocal of the original fraction. Dividing by a fraction is the same as multiplying by its reciprocal.
To convince yourself that this is true, try two additional examples on the calculator. Divide the first fraction by the second, and note your answer. Then, multiply the first fraction by the reciprocal of the second fraction.
Remember to put each fraction in parentheses. Watch the screen at the top as you input the calculation to make sure that you are entering it correctly.
(a) The calculator doesn’t enter the division very elegantly, but you can tell that it is correct because the middle fraction line is longer than the other two. This means that the whole of the first fraction is divided by the whole of the second fraction.
Multiplying by the reciprocal gives the same answer:
(b) The division results in:
Meanwhile, multiplication by the reciprocal gives:
- To divide fractions on the calculator, put each fraction in a set of parentheses.
- To divide by a fraction, multiply by its reciprocal.