Direct and Indirect Proportion

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

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

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 Income80

Nr of CDs Income The Answer…

Now let’s graph it:

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.

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.

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 Cost p/p42

The Answer… Nr of People Cost p/p ,508,405,25

Now let’s graph it: