## Quantities in Direct and Inverse Proportion

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,

and for the second graph

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

We are typically asked something like:

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

whenFind

a)when

b)when

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

c)Use you equation to findwhen

a)

b)

c)so