Quadratic Functions (DP IB Analysis & Approaches (AA)): Revision Note

Did this video help you?

Quadratic Functions & Graphs

What are the key features of quadratic graphs?

  • A quadratic graph can be written in the form y equals a x squared plus b x plus c where a not equal to 0

  • The value of a affects the shape of the curve

    • If a is positive the shape is concave up

    • If a is negative the shape is concave down

  • The y-intercept is at the point (0, c)

  • The zeros or roots are the solutions to a x squared plus b x plus c equals 0

    • These can be found by

      • Factorising

      • Quadratic formula

      • Using your GDC

    • These are also called the x-intercepts

    • There can be 0, 1 or 2 x-intercepts

      • This is determined by the value of the discriminant

  • There is an axis of symmetry at x equals negative fraction numerator b over denominator 2 a end fraction

    • This is given in your formula booklet

    • If there are two x-intercepts then the axis of symmetry goes through the midpoint of them

  • The vertex lies on the axis of symmetry

    • It can be found by completing the square

    • The x-coordinate is x equals negative fraction numerator b over denominator 2 a end fraction

    • The y-coordinate can be found using the GDC or by calculating y when x equals negative fraction numerator b over denominator 2 a end fraction

    • If a is positive then the vertex is the minimum point

    • If a is negative then the vertex is the maximum point

Quadratic Graphs Notes Diagram 1
Quadratic Graphs Notes Diagram 2

What are the equations of a quadratic function?

  • space f left parenthesis x right parenthesis equals a x squared plus b x plus c

    • This is the general form

    • It clearly shows the y-intercept (0, c)

    • You can find the axis of symmetry by x equals negative fraction numerator b over denominator 2 a end fraction

      • This is given in the formula booklet

  • space f left parenthesis x right parenthesis equals a left parenthesis x minus p right parenthesis left parenthesis x minus q right parenthesis

    • This is the factorised form

    • It clearly shows the roots (p, 0) & (q, 0)

    • You can find the axis of symmetry by x equals fraction numerator p plus q over denominator 2 end fraction

  • space f left parenthesis x right parenthesis equals a left parenthesis x minus h right parenthesis squared plus k

    • This is the vertex form

    • It clearly shows the vertex (h, k)

    • The axis of symmetry is therefore x equals h

    • It clearly shows how the function can be transformed from the graph y equals x squared

      • Vertical stretch by scale factor ­a

      • Translation by vector stretchy left parenthesis table row h row k end table stretchy right parenthesis

How do I find an equation of a quadratic?

  • If you have the roots x = p and x = q...

    • Write in factorised form space y equals a left parenthesis x minus p right parenthesis left parenthesis x minus q right parenthesis

    • You will need a third point to find the value of a

  • If you have the vertex (h, k) then...

    • Write in vertex form y equals a left parenthesis x minus h right parenthesis squared plus k

    • You will need a second point to find the value of a

  • If you have three random points (x1, y1), (x2, y2) & (x3, y3) then...

    • Write in the general form y equals a x squared plus b x plus c

    • Substitute the three points into the equation

    • Form and solve a system of three linear equations to find the values of a, b & c

Examiner Tips and Tricks

  • Use your GDC to find the roots and the turning point of a quadratic function

    • You do not need to factorise or complete the square

    • It is good exam technique to sketch the graph from your GDC as part of your working

Worked Example

The diagram below shows the graph of space y equals f left parenthesis x right parenthesis, where space f left parenthesis x right parenthesis is a quadratic function.

The intercept with the y-axis and the vertex have been labelled.

2-2-1-ib-aa-sl-we-image

Write down an expression for space y equals f left parenthesis x right parenthesis.

2-2-1-ib-aa-sl-quad-function-we-solution
👀 You've read 1 of your 5 free revision notes this week
An illustration of students holding their exam resultsUnlock more revision notes. It's free!

By signing up you agree to our Terms and Privacy Policy.

Already have an account? Log in

Did this page help you?

Dan Finlay

Author: Dan Finlay

Expertise: Maths Subject Lead

Dan graduated from the University of Oxford with a First class degree in mathematics. As well as teaching maths for over 8 years, Dan has marked a range of exams for Edexcel, tutored students and taught A Level Accounting. Dan has a keen interest in statistics and probability and their real-life applications.

Download notes on Quadratic Functions