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Identifying features of a quadratic functionFinding the turning point and the line of symmetry

The key features of a quadratic function are the y-intercept, the axis of symmetry, and the coordinates and nature of the turning point (or vertex).

Part of MathsAlgebraic skills

Finding the turning point and the line of symmetry

The coordinates of the turning point and the equation of the line of symmetry can be found by writing the quadratic expression in completed square form.

Example

Find the equation of the line of symmetry and the coordinates of the turning point of the graph of \(y = x^2 鈥 6x + 4\)

The coefficient of \(x^2\) is positive, so the graph will be a positive U-shaped curve.

Writing \(y = x^2 鈥 6x + 4 \) in completed square form gives \(y = (x 鈥 3)^2 鈥 5\)

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 the graph \(y = x^2 鈥6x + 4\) is given when \(x = 3\)

\(x = 3\) is also the equation of the line of symmetry

When \(x = 3\), \(y = -5\) so the turning point has coordinates (3, -5)

Question

Sketch the graph of \(y = x^2 鈥 2x 鈥 3\), labelling the points of intersection and the turning point.