OCR A Level Maths: Pure

Revision Notes

7.5.1 Implicit Differentiation

Implicit Differentiation

What is implicit differentiation?

  • An equation connecting x and y is not always easy to write explicitly in the form y= f(x) or x = f(y)
  • However you can still differentiate such an equation implicitly using the chain rule:

 Implicit Diff Illustr 1_form, AS & A Level Maths revision notes

 Implicit Diff Illustr 1_ex, AS & A Level Maths revision notes

  • Combining this with the product rule gives us:

 Implicit Diff Illustr 2_form, AS & A Level Maths revision notes

 

Implicit Diff Illustr 2_ex, AS & A Level Maths revision notes

  • These two special cases are especially useful:

 

Implicit Diff Illustr 3, AS & A Level Maths revision notes

 

  • When x and y are connected in an equation you can differentiate both sides with respect to x and rearrange to find a formula (usually in terms of x and y ) for dy/dx
    • Note that dy/dx is a single algebraic object
    • When rearranging do not treat dy/dx as a fraction
    • Especially do not try to separate dy and dx and treat them as algebraic objects on their own!

Implicit Diff Illustr 4, AS & A Level Maths revision notes

Exam Tip

  • When using implicit differentiation you will not always be able to write dy/dx simply as a function of x.
  • However, this does not stop you from answering questions involving the derivative.

Worked example

Implicit Diff Example, AS & A Level Maths revision notes

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Roger

Author: Roger

Roger's teaching experience stretches all the way back to 1992, and in that time he has taught students at all levels between Year 7 and university undergraduate. Having conducted and published postgraduate research into the mathematical theory behind quantum computing, he is more than confident in dealing with mathematics at any level the exam boards might throw at you.