# Sketching graphs

Plotting a graph takes time. Often mathematicians just want to know the key features.

These are: shape, location and some key points (such as where the graph crosses the axes or turning points).

Therefore, aim to recognise the equations and graphs of quadratics, cubics, reciprocals, exponentials and circles.

## Sketching a quadratic graph using factors

If a quadratic equation can be factorised, the factors can be used to find where the graph crosses the -axis.

### Example

Sketch .

The quadratic factorises to give so the solutions of the equation are and .

The graph of crosses the -axis at and .

The coefficient of is positive, so the graph will be a positive U-shaped curve with a turning point and line of symmetry at .

To find where the graph crosses the -axis, work out when : so the graph crosses the -axis at .

### Example

Sketch .

The quadratic factorises to give so the only solution of the equation is .

The graph of touches the -axis at .

The coefficient of is positive, so the graph will be a positive U-shaped curve.

To find where the graph crosses the -axis, work out when : so the graph crosses the -axis at .

## Sketching a quadratic using the turning point and the line of symmetry - Higher

The coordinates of the turning point and the equation of the line of symmetry can be found by completing the square for the quadratic equation.

### Example

Sketch .

The coefficient of is positive, so the graph will be a positive U-shaped curve.

Writing in completed square form gives .

Squaring positive or negative numbers always gives a positive value. The lowest value given by a squared term is 0, which means that the turning point of is when . This also gives the equation of the line of symmetry for the quadratic graph.

The value of when is -5. This value is always the same as the constant term in the completed square form of the equation.

So the graph of has a line of symmetry with equation and a minimum turning point at (3, -5).

When , . So the graph crosses the -axis at (0, 4).

Question

Sketch the graph of , labelling the points of intersection and the turning point.

The coefficient of is positive, so the graph will be a positive U-shaped curve.

Factorising gives and so the graph will cross the -axis at and .

The graph will cross the -axis at (0, -3).

Writing in completed square form gives , so the coordinates of the turning point are (1, -4).

The turning point could also be found by using symmetry as it will have an value halfway between and , so when .

Move on to Test