Derivative of a Quotient of Functions
Division within derivatives is more complicated than the other rules we've seen so far. Make some space in the ol' memory bank for the Quotient Rule. The Quotient Rule states that the derivative of the function is
The quotient rule is more complicated than the product rule. Here are some important bits that need to be remembered.
- The numerator involves subtraction. Not addition, but subtraction:
- The denominator is the square of the original function's denominator:
- The numerator is
f 'g – fg',
not the other way around.
One way to remember this is that we read the numerator of a fraction first, and in the quotient rule we take the derivative of the numerator first:
There's also a mnemonic that may be helpful. If we think of the original function as
then the numerator in the quotient rule is
"low dee-high minus high dee-low," where "dee" means "derivative." The phrase does have a nice ring to it.
"low dee-high minus high dee-low" translates to
gf ' – fg',
but this is the same thing as
f 'g – fg'.
That's a lot of stuff to remember, but practice will make it easier. After we learn the Chain Rule we'll be able to re-create the Quotient Rule.
If a function is written as a fraction, it doesn't necessarily mean we need to use the quotient rule to find the derivative. If the denominator of a function is a constant, we can rewrite the function and avoid using the quotient rule.