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Differentiation (CIE IGCSE Maths: Extended)

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Jamie W

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Jamie W

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Differentiation

What is differentiation?

  • Differentiation is part of the branch of mathematics called Calculus
  • It is concerned with the rate at which changes takes place – so has lots of real‑world uses:
    • The rate at which a car is moving (its speed)
    • The rate at which a virus spreads amongst a population

 Diff Basics Notes fig1, downloadable IGCSE & GCSE Maths revision notes 

  • To begin to understand differentiation you’ll need to understand gradients

How are gradients related to rates of change?

  • Gradient generally means steepness.
    • For example, the gradient of a road up the side of a hill is important to lorry drivers

 Diff Basics Notes fig2, downloadable IGCSE & GCSE Maths revision notes

On a graph the gradient refers to how steep a line or a curve is

  • It is really a way of measuring how fast y changes as x changes
  • This may be referred to as the rate at which y

  • So gradient describes the rate at which change happens

How do I find the gradient of a curve using its graph?

  • For a straight line the gradient is always the same (constant)
    • Recall y = mx + c, where m is the gradient

 Diff Basics Notes fig3, downloadable IGCSE & GCSE Maths revision notes

  • For a curve the gradient changes as the value of x changes
  • At any point on the curve, the gradient of the curve is equal to the gradient of the tangent at that point
    • A tangent is a straight line that touches the curve at one point

Diff Basics Notes fig4, downloadable IGCSE & GCSE Maths revision notes 

How do I find the gradient of a curve using algebra? 

  • This is really where the fun begins!
    • Drawing tangents each time you want the gradient of a curve is too much effort
    • It would be great if you could do it using algebra instead
  • The equation of a curve can be given in the form y equals straight f open parentheses x close parentheses
    • Inputting x-coordinates gives outputs of y-coordinates
  • It is possible to create an algebraic function that take inputs of x-coordinates and gives outputs of gradients
    • All of this is done without needing to sketch any graphs
  • This type of function has a few commonly used names:
    • The gradient function
    • The derivative
    • The derived function
  • The way to write this function is fraction numerator straight d y over denominator straight d x end fraction
    • This is pronounced "dy by dx"
    • In function notation, it can be written straight f apostrophe open parentheses x close parentheses
      • pronounced f-dashed-of-x
  • To get from y equals straight f open parentheses x close parentheses to fraction numerator straight d y over denominator straight d x end fraction equals straight f apostrophe open parentheses x close parentheses you need to do an operation called differentiation
    • Differentiation turns curve equations into gradient functions
  • The main rule for differentiation is shown

 

Diff Basics Notes fig5, downloadable IGCSE & GCSE Maths revision notes

  • This looks worse than it is!
  • For powers of x

STEP 1   Multiply the number in front by the power

STEP 2   Take one off the power (reduce the power by 1)

  • 2x6 differentiates to 12x5
    • Note the following:
      • kx differentiates to k
      • so 10x differentiates to 10
      • any number on its own differentiates to zero
      • so 8 differentiates to 0

 Diff Basics Notes fig6, downloadable IGCSE & GCSE Maths revision notes

How do I use the gradient function to find gradients of curves?

  • Find the x-coordinate of the point on the curve you're interested in
  • Use differentiation to find the gradient (derived) function, fraction numerator straight d y over denominator straight d x end fraction
  • Substitute the x-coordinate into the gradient (derived) function to find the gradient

 Diff Basics Notes fig7, downloadable IGCSE & GCSE Maths revision notes

Examiner Tip

  • When differentiating long awkward expressions, write each step out fully and simplify the numbers after
  • Don't forget to write the left-hand sides of y = .... and fraction numerator straight d y over denominator straight d x end fraction = ... to avoid mixing up the curve equation with the gradient function

Worked example

Diff Basics Example fig1, downloadable IGCSE & GCSE Maths revision notes

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Jamie W

Author: Jamie W

Expertise: Maths

Jamie graduated in 2014 from the University of Bristol with a degree in Electronic and Communications Engineering. He has worked as a teacher for 8 years, in secondary schools and in further education; teaching GCSE and A Level. He is passionate about helping students fulfil their potential through easy-to-use resources and high-quality questions and solutions.