### Video Transcript

The product rule says that the
derivative of ππ is equal to the derivative of π times π plus π times the
derivative of π. Use this to derive a formula for
the derivative of π times π times β.

In this question, weβve been given
the product rule and asked to use it to find a formula for the derivative of the
product of three functions. These are π, π, and β. Weβre going to begin by splitting
π times π times β up. Weβre going to write it as ππ
times β. Remember since multiplication is
commutative, we could have alternatively written it as π times πβ and we will get
the same answer either way. So we can say that the derivative
of ππβ is equal to the derivative of ππ times β.

And weβre now going to apply the
product rule. We can see that this is equal to
the derivative of ππ times β plus ππ times the derivative of β. And now, we spot that the first
term we have is the derivative of ππ. We know though by the definition of
the product rule that this is the same as the derivative of π times π plus π
times the derivative of π. So we replace this in our
formula. And weβre going to distribute these
parentheses.

When we do, we see that the formula
for the derivative of ππβ is the derivative of π times π times β plus π times
the derivative of π times β plus π times π times the derivative of β. You might also like to see if you
can apply this idea to help you find a formula for the derivative of the product of
four functions, say ππβπ.