remember the definition of the derivative of a function (if it exists):

or alternatively,

use as your function in either of the expressions above to find the derivative by defintion

the quotient rule gives the derivative of rational functions, that is, functions of the form , where are functionsb) Use part a) and the product rule to prove the quotient rule.

note that this can be written as

so you can use the product rule to find the derivative of this product. note that you found the formula to differentiate things like in part (a)