Reverse Chain Rule
What is the reverse chain rule?
- The Chain Rule is a way of differentiating two (or more) functions
- The Reverse Chain Rule (RCR) refers to integrating by inspection
- Spotting that chain rule would be used in the reverse (differentiating) process
How do I know when to use the reverse chain rule?
- The reverse chain rule is used when we have the product of a composite function and the derivative of its second function
- Integration is trickier than differentiation; many of the shortcuts do not work
- For example, in general
- However, this result is true if is linear
- Formally, in function notation, the reverse chain rule is used for integrands of the form
-
- This does not have to be strictly true, but ‘algebraically’ it should be
- If the coefficients do not match ‘adjust and compensate’ can be used
- For example, differentiates to with the chain rule
- so with the reverse chain rule
- But to do we need to:
- Take out the five:
- Force a 2 inside (adjust) and divide the outside by a 2 (compensate):
- The bit inside the integral is now a reverse chain rule
- The answer is
- For example, differentiates to with the chain rule
- A particularly useful instance of the reverse chain rule to recognise is
-
- i.e. the numerator is (almost) the derivative of the denominator
- 'adjust and compensate' may need to be used to deal with any coefficients
- e.g.
Examiner Tip
- You can always check your work by differentiating, if you have time
Worked example
A curve has the gradient function.
Given that the curve passes through the point, find an expression for.
Write f(x) as an integral.
Take 5 out of the integral as a factor.
The main function is sin(...), which would have come from -cos(...).
Adjust and compensate the coefficients.
2x3 would differentiate to 6x2 so -cos(2x3) would differentiate to (6x2)sin(2x3)
Integrate.