Graphing direct proportion | Maths | Quizalize & Oak National Academy

Starter quiz

Two variables are in direct ______ if they have a constant multiplicative relationship.
- 'proportion' ✓
Which of the following show a directly proportional relationship between A and B?
A and B are directly proportional. The missing value of B is ______.
- '36' ✓
A and B are directly proportional. The missing value of B is ______.
- '90' ✓
The graph shows the relationship between cost and number of boxes bought. Can you spend exactly £5 on boxes?
- No, reading off the graph at £5 shows you can buy between 2 and 3 boxes. ✓
- Yes, reading off the graph at £5 shows you can buy exactly 3 boxes.
This graph shows that A and B are in direct proportion. The proportional relationship between A and B is B = A × ______.
- '3' ✓

Exit quiz

Graphs showing direct proportion have a positive gradient with an ______ of 0.
- 'intercept' ✓
Which of the following graphs does NOT show direct proportion?
Which tables show that A and B are in direct proportion?
The constant of proportionality represented by this graph is ______.
- '4' ✓
Which of the following is NOT true of this graph?
- The constant of proportionality is $3$ .
- The equation of the line is $x = 3y$ . ✓
- The gradient of the line is $3$ .
- The graph has an intercept of zero.
- The graph does not show a directly proportional relationship. ✓
What are the key features of a graph showing direct proportion?
- It has a negative gradient.
- It has an intercept of zero. ✓
- It has a constant positive gradient. ✓
- The intercept can be 0.1
- It has a gradient of 0

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Lesson Details

Key learning points

Direct proportion can be recognised graphically.
A direct proportion graph is always a straight line.
The origin is always a point on a direct proportion graph.
The gradient tells us the constant of proportionality.
Using these features it is possible to identify direct proportion graphs.

Common misconception

Assuming that any graph with a constant positive gradient represents variables that are in direct proportion.

Encourage the useful check of reading a pairs of values from the graph, doubling one of them and checking the graph to see if the other variable has doubled. If it has it shows a directly proportional relationship if not it doesn't.

Keywords

Direct proportion - Two variables are in direct proportion if they have a constant multiplicative relationship.