Edexcel IGCSE Maths

Revision Notes

3.11.2 Differentiation - Turning Points

Remind me of the rule for differentiating powers

  • If y = kxn then dy/dx = knxn-1

 

Rule for IGCSE Diff, downloadable IGCSE & GCSE Maths revision notes

What is a turning point?

  • The easiest way to think of a turning point is that it is a point at which a curve changes from moving upwards to moving downwards, or vice versa
  • Turning points are also called stationary points
  • Ensure you are familiar with Differentiation – Basics before moving on

 

Turn Pts Notes fig1, downloadable IGCSE & GCSE Maths revision notes

  • At a turning point the gradient of the curve is zero
    • If a tangent is drawn at a turning point it will be a horizontal line
    • Horizontal lines have a gradient of zero
  • This means at a turning point the derivative or gradient function equals zero

 

Turn Pts Notes fig2, downloadable IGCSE & GCSE Maths revision notes

How do I know if a curve has turning points?

  • You can see from the shape of a curve whether it has turning points or not
  • At IGCSE, two types of turning point are considered:
    • Maximum points – this is where the graph reaches a “peak
    • Minimum points – this is where the graph reaches a “trough

 

Turn Pts Notes fig3, downloadable IGCSE & GCSE Maths revision notes

 

  • These are sometimes called local maximum/minimum points as other parts of the graph may still reach higher/lower values

How do I find the coordinates of a turning point?

  • STEP 1  Solve the equation of the gradient function (derivative) equal to zero
    ie. solve dy/dx = 0
    This will find the x-coordinate of the turning point
  • STEP 2  To find the y-coordinate substitute the x-coordinate into the equation of the graph
    ie. substitute x into “y = …

 

Turn Pts Notes fig4, downloadable IGCSE & GCSE Maths revision notes

How do I know which point is a maximum and which is a minimum?

  • The easiest way to do this is to recognise the shape of the curve
    • … either from a given sketch of the curve
    • … a sketch of the curve you can quickly draw yourself
      (You may even be asked to do this as part of a question)
    • … the equation of the curve
  • For parabolas (quadratics) it should be obvious …
    • … a positive parabola (positive x2 term) has a minimum point
    • … a negative parabola (negative x2 term) has a maximum point

 

Turn Pts Notes fig5, downloadable IGCSE & GCSE Maths revision notes

  • Cubic graphs are also easily recognisable …
    • … a positive cubic has a maximum point on the left, minimum on the right
    • … a negative cubic has a minimum on the left, maximum on the right

 

Turn Pts Notes fig6, downloadable IGCSE & GCSE Maths revision notes

Exam Tip

Read questions carefully – sometimes only the x-coordinate of a turning point is required.

Differentiating accurately is crucial in leading to equations you can work with and solve.

Worked Example

Turn Pts Example fig1 qu, downloadable IGCSE & GCSE Maths revision notes

Turn Pts Example fig2 sol, downloadable IGCSE & GCSE Maths revision notes

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