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# Partial derivatives, introduction

Video transcript

- [Voiceover] So, let's say I have some multi-variable function like F of XY. So, they'll have a two variable input, is equal to, I don't know, X squared times Y, plus sin(Y). So, allow for just a single number. It's a scalar valued function. Question is, how do we take the derivative of an expression like this? And there's a certain method
called a partial derivative, which is very similar
to ordinary derivatives and I kinda wanna show
how they're secretly the same thing. So, to do that, let me
just remind ourselves of how we interpret the notation
for ordinary derivatives. So, if you have something
like F(X)=X squared, and let's say you wanna
take its derivative, and I'll live nets notation here, df/dx, and let's evaluate it at two, let's say. I really like this notation
because it's suggestive of what's going on. If we sketch out a graph, so this axis represents our output, this over here represents our input, and X squared has a certain
parabolic shape to it. Something like that. Then we go to the input,
x equals one, two. This little DX here, I like to interpret as just a little nudge in the X direction. And it's kind of the size of that nudge. And then DF, is the resulting change in the output after you make that initial little nudge. So, it's this resulting change. And when you're thinking
n terms of graphs, this is slope. You kind of have this rise over run for your ratio between the tiny change of the output that's caused
by a tiny change in the input. And of course, this is
dependent on where you start. Over here we have X=2. But, you could also think
about this without graphs if you really wanted to. You might just think about, your input space as just a number line and your output space,
also as just a number line, the output of F over here. And really, you're just thinking
of somehow mapping numbers from here onto the second line. And in that case, your initial nudge, your initial little DX, would be some nudge on that number line. And you're wondering how that influences the function itself. So maybe that causes a nudge
that's four times as big and that would mean your
derivative is four at that point. So, the reason I'm talking about this is because over in the multi-variable world, we can pretty much do the same thing. You can write df/dx and
interpret that as saying, hey how does a tiny change in
the input in the X direction influence the output. But, this time, the way
that you might visualize it, you'd be thinking of your input space. Here, I'll draw it down
here, as the XY plane. So, this time, this is not
gonna be graphing the function, this is, every point on
the plane is an input. And let's say you were evaluating this at a point like one, two. In that case, you'd go over
to the input that's one and then two, and then you'd say, okay, so this tiny nudge in the input, this tiny change DX, how does
that influence the output. And in this case the output, I mean, it's still just a number. So maybe we go off to the side here and we draw just like a
number line as our output. And somehow we're thinking
about the function as mapping points on the
plane to the number line. So you'd say, okay, that's your DX, how much does it change the output? And, maybe this time it
changes it negatively. It depends on your function. And that would be your DF. And you can also do this
with the Y variable, right. There's no reason that
you can't say DF, DY, and evaluate at that same point, one, two. And interpret totally the same way. Except, this time your DY would be a change in the Y direction. So maybe I should really
emphasize here that that DX is a change in
the X direction here and that DY is a change
in the Y direction. And maybe, when you change
your F according to Y, it does something different. Maybe, the output increases
and it increases by a lot, it's more sensitive to Y. Again, it depends on the function. And I'll show you how you can
compute something like this in just a moment here. But, first there's kind of
an annoying thing associated with partial derivatives,
where we don't write them with D's in DX/DF. People came up with this new notation, mostly just to emphasize to the reader of your equation that it's a multi-variable function involved. And what you do, is
you say, you write a D, but it's got kind of a curl at the top. It's this new symbol and
people will often read it as partial. So, you might read like
partial F, partial Y. If you're wondering, by the way, why we call these partial derivatives, it's sort of like, this
doesn't tell the full story of how F changes 'cause it only
cares about the X direction. Neither does this, this only
cares about the Y direction. So, each one is only a
small part of the story. So, let's actually evaluate
something like this. I'm gonna go ahead and
clear the board over here. I think the one-dimensional analogy is something we probably have already. So, little remnants. So, if you're actually
evaluating something like this, here, I'll write it up here again up here. Partial derivative of
F, with respect to X, and we're doing it at one, two. It only cares about
movement in the X direction, so it's treating Y as a constant. It doesn't even care about
the fact that Y changes. As far as it's concerned,
Y is always equal to two. So, we can just plug
that in ahead of time. So, I'm gonna say partial, partial X, this is another way you might write it, put the expression in here. And I'll say X squared,
but instead of writing Y, I'm just gonna plug in that
constant ahead of time. 'Cause when you're only
moving in the X direction, this is kind of how the
multi-variable function sees the world. And I'll just keep a little note that we're evaluating this
whole thing at X equals one. And here, this is actually
just an ordinary derivative. This is an expression that's an X, you're asking how it changes as you shift around X and
you know how to do this. This is just taking the derivative of X square times two is gonna be 4x 'cause
X squared goes to 2x. And then the derivative of a constant, sin of two is just a constant, is zero. And of course we're evaluating
this at X equals one, so your overall answer is gonna be four. And as for practice, let's also do that with derivative with respect to Y. So, we look over here, I'm
gonna write the same thing. You're taking the partial derivative of F with respect to Y. We're evaluating it at
the same point one, two. This time it doesn't care about
movement in the X direction. So, as far as it's concerned, that X just stays constant at one. So, we'd write one squared
times Y, plus sin(Y). Sin(Y). And you're saying, oh, I'm keeping track of this at Y=2. So, it's kind of, you're
evaluating at Y=2. When you take the
derivative, this is just 1xY. So the derivative is one. This over here, the
derivative is cosine, cos(Y). Again, we're evaluating
this whole thing at Y=2. So, you're overall answer, it would be 1+cos(2). I'm not sure what the value of cos(2) is off the top of my head, but that would be your answer. And, this is a partial
derivative at a point, but a lot of times, you're
not asked to just compute it at a point, what you
want is a general formula that tells you, hey, plug in any point XY and it should spit out the answer. So, let me just kinda go
over how you would do that. It's actually very similar, but this time, instead of
plugging in the constant ahead of time, we just have to pretend that it's a constant. So let me make a little bit
of space for ourselves here. We don't need any of this anymore. I'm gonna leave the partial
partial F, partial partial Y. We want this as a more
general function of X and Y. Well we kind of do the same thing. We're gonna say that this is derivative with respect to X, and I'm using partials
just to kind of emphasize that it's a partial derivative. And now, we'd write X squared and then kind of emphasize that
it's a constant value of Y, plus the sin, and again, I'll say Y. And here, I'm writing the variable Y, but we have to pretend
like it's a constant, you're pretending that you plugin two or something like that. And you still just take the derivative. So, in this case, the derivative of X
squared times a constant, is just 2x times that constant. And over here, the derivative
of a constant is always zero. So that's just always gonna be zero. So, this is your partial derivative as a more general formula. If you plugged in one, two to this, you'd get what we had before. And similarly, if you're doing this with partial F partial Y, we write down all of the same things, now you're taking it with respect to Y. And I'm just gonna copy
this formula here actually. But this time, we're considering all of the the X's to be constants. So, in this case, when
you take the derivative with respect to Y of
some kind of constant, constant squared is a constant, times Y, it's just gonna
equal that constant. So, this is gonna be X squared. And over here, you're taking
the derivative of sin(Y). There's no X's in there,
so that remains the sin(Y). Now this is a more general formula. If you plugged in one,
two, you would get one. I'm sorry that's cos(y). Cos(y) because we're taking a derivative. So, if you plugged in one, two, you would get 1+cos(1), which is what we had before. So this, this is really what you'll see for how to compute a partial derivative. You pretend that one of
the variables is constant and you take an ordinary derivative. And in the back of your mind, you're thinking this is because you're just
moving in one direction for the input and you're seeing
how that influences things. And then, you might move in
one direction for another input and see how that influences things. In the next video, I'll
show you what this means in terms of graphs and slopes, but it's important to understand
that graphs and slopes are not the only way to
understand derivatives because as soon as you start thinking about dector valued functions or functions with inputs of
higher dimensions than just two, you can no longer think
in terms of graphs. But, this idea of nudging
the input in some direction, seeing how that influences the output, and then taking the ratio
of that output nudge to the input nudge, that's a more general
way of viewing things. And that's gonna be very
helpful moving forward in multi-variable cap.