The two graphs showquantities directly proportional in the first graph and inverselyproportional in the second graph.

For the first graph we canwrite down,

and for the second graph

where, in each case,isa constant of proportionality.

We are typically askedsomething like:

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

whenFind

a)when

b)when

Answers

a)

b)

Example: The intensity oflightfroma bulb is inversely proportional to the distance

a)Write down an expressionrelatingand

Ifwhen,find an equation betweenand

b)Use your equation tofindwhen

c)Use you equation tofindwhen

Answers

a)

b)

c)so