1. Direct and Indirect Proportion

2. What is Direct Proportion? If a quantity ‘A’ is directly proportional to another quantity ‘B’, this means that if ‘A’ increases, then ‘B’ increases by the same amount The ratio of A:B is unchanged

3. Direct Proportion Example The cost of petrol depends on the number of litres you buy. If you buy 0l then the cost will be R0. The litres is the independent variable The cost is the dependent variable

4. Try this… A performer sells his CDs for R80. His income equals: y = 80 x where y is the cost and x the number of CD’s Complete the table to represent the relationship. Nr of CDs 1234510 Income80

5. Nr of CDs 1234510 Income80160240320400800 The Answer…

6. Now let’s graph it:

7. What is Indirect Proportion? If a quantity ‘A’ is inversely proportional to another quantity ‘B’, then as ‘A’ increases ‘B’ decreases by the same amount.

8. Indirect Proportion Example Speed (S) vs Time (T) Speed is inversely proportional to time; if you double your speed (S x 2), then you halve the time (T x ½) it takes to travel the same distance.

9. Try this… A school hires a bus for R420. It takes a maximum of 80 passengers. Complete the table to represent the relationship. To calculate the cost per person, we can say: y = 420 ÷ x where y is the cost pp and x the number of people Nr of People 102030405080 Cost p/p42

10. The Answer… Nr of People 102030405080 Cost p/p42211410,508,405,25

11. Now let’s graph it: