- [Instructor] So I have two different expressions here
that I wanna take the derivative of.
And what I want you to do is pause the video
and think about how you would first approach
taking the derivative of this expression
and how that might be the same or different
as your approach in taking the derivative
of this expression.
The goal here isn't to compute the derivatives all the way,
but really to just think about how we identify
what strategies to use.
Okay so let's first tackle this one.
And the key when looking at a complex expression
like either of these is to look at the big picture
structure of the expression.
So one way to think about it is,
let's look at the outside rather than the inside details.
So if we look at the outside here,
we have the sine
of something.
So there's a sine of something going on here
that I'm going to circle in red
or in this pink color.
So that's how my brain thinks about it.
From the outside I'm like okay,
big picture I'm taking the sine
of some stuff.
I might be taking some stuff to some exponent.
In this case, I'm inputting in a trigonometric expression.
But if you have a situation like that,
it's a good sign that
the chain rule is in order.
So let me write that down.
So we would wanna use in this case
the chain rule, C.R. for chain rule.
And how would we apply it?
Well we would take the derivative of the outside
with respect to this inside
times the derivative of this inside
with respect to x.
And I'm gonna write it the way that my brain
sometimes thinks about it.
So we can write this as
the derivative
with respect to
that something,
and I'm just gonna make that pink circle
for the something rather than writing it all again,
of sine of that something,
sine of that something,
not even thinking about what that something is just yet,
times, times the derivative
with respect to x
of that something.
This is just an application
of the chain rule.
No matter what was here in this
pink colored circle,
it might have been something with square roots
and logarithms and whatever else,
as long as it's being contained within the sine,
I would move to this step.
The derivative with respect to that something
of sine of that something
times the derivative with respect to x of the something.
Now what would that be tangibly in this case?
Well this first part,
I will do it in orange,
this first part would just be cosine
of x squared plus five
times cosine of x.
So that's that circle right over there.
Let me close the cosine right over there.
And then times the derivative with respect to x,
times the derivative with respect to x,
of all of this again,
of x squared plus five
times cosine of x.
And then I would close my brackets.
And of course I wouldn't be done yet,
I have more derivative taking to do.
Here now I would look at the big structure
of what's going on,
and I have two expressions being multiplied.
I don't have just one
big expression that's being an input
into like a sine function or cosine function
or one big expression that's taken to some exponent.
I have two expressions being multiplied.
I have this
being multiplied by this.
And so if I'm just multiplying two expressions,
that's a pretty good clue that to compute this part,
I would then use the product rule.
And I could keep doing that and compute it,
and I encourage you to do so,
but this is more about the strategies
and how do you recognize them.
But now let's go to the other example.
Well this looks a lot more like this step
of the first problem than the beginning
of the original problem.
Here I don't have a sine of a bunch of stuff
or a bunch of stuff being raised to one exponent.
Here I have the product of two expressions
just like we saw over here.
We have this expression
being multiplied by this expression.
So my brain just says okay I have two expressions,
then I'm going to use the product rule.
Two expressions being multiplied,
I'm going to use the product rule.
If it was one expression being divided
by another expression, then I would use the quotient rule.
But in this case it's going to be the product rule.
And so that tells me
that this is going to be the derivative with respect to x
of the first expression,
just gonna do that with the orange circle,
times the second expression,
I'm gonna do that with the blue circle,
plus the first expression, not taking its derivative,
the first expression, times
the derivative with respect to x of,
of the second expression.
Once again here,
this is just the product rule.
You can substitute sine of x squared plus five
where you see this orange circle,
and you can substitute cosine of x
where you see this blue circle.
But the whole point here isn't to actually solve this
or compute this,
but really to just show
how you identify the structures in these expressions
to think about well do I use the chain rule first
and then use the product rule here?
Or in this case do I use the product rule first?
And even once you do this,
you're not going to be done.
Then to compute this derivative,
you're going to have to use the chain rule,
and you'll keep going until you don't have any more
derivatives to take.