The two graphs show quantities directly proportional in the first graph and inversely proportional in the second graph.

For the first graph we can write down for the first graph,

and for the second graphover

where, in each case,is a constant of proportionality.

We are typically asked:

The amount of petrolused by a car is directly proportional to the distance travelled,

p=50whend=8.Find

a)when

b)when

Answers

a)

b)

Example: The intensity of lightfrom a bulb is inversely proportional to the distance

a)Write down an expression relatingand

Ifwhen, find an equation betweenand

b)Use your equation to findwhen

c)Use you equation to findwhen

Answers

a)

b)

c)so