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# Differentiating polynomials

Video transcript

Now that we know the power
rule, and we saw that in the last video, that
the derivative with respect to x, of x to the n, is going to
be equal to n times x to the n minus 1 for n not equal 0. I thought I would expose you
to a few more rules or concepts or properties of derivatives
that essentially will allow us to take the derivative
of any polynomial. So this is powerful
stuff going on. So the first thing I
want to think about is, why this little special
case for n not equaling 0? What happens if n equals 0? So let's just think
of the situation. Let's try to take the
derivative with respect to x of x to the 0 power. Well, what is x to the
0 power going to be? And we can assume that x for
this case right over here is not equal to 0. 0 to the 0, weird things
happen at that point. But if x does not
equal 0, what is x to the 0 power going to be? Well, this is the same thing
as the derivative with respect to x of 1. x to the 0 power is
just going to be 1. And so what is the derivative
with respect to x of 1? And to answer that question,
I'll just graph it. I'll just graph f of
x equals 1 to make it a little bit clearer. So that's my y-axis. This is my x-axis. And let me graph y equals
1, or f of x equals 1. So that's 1 right
over there. f of x equals 1 is just
a horizontal line. So that right over
there is the graph, y is equal to f of x,
which is equal to 1. Now, remember the derivative,
one way to conceptualize is just the slope of the
tangent line at any point. So what is the slope of the
tangent line at this point? And actually, what's the
slope at every point? Well, this is a line, so
the slope doesn't change. It has a constant slope. And it's a completely
horizontal line. It has a slope of 0. So the slope at every
point over here, slope is going to be equal to 0. So the slope of this
line at any point is just going to be equal to 0. And that's actually going
to be true for any constant. The derivative, if I had a
function, let's say that f of x is equal to 3. Let's say that's
y is equal to 3. What's the derivative
of y with respect to x going to be equal to? And I'm intentionally showing
you all the different ways of the notation for derivatives. So what's the derivative
of y with respect to x? It can also be
written as y prime. What's that going
to be equal to? Well, it's the slope
at any given point. And you see that no matter
what x you're looking at, the slope here is going to be 0. So it's going to be 0. So it's not just x to the 0. If you take the derivative
of any constant, you're going to get 0. So let me write that. Derivative with respect
to x of any constant-- so let's say of a where this
is just a constant, that's going to be equal to 0. So pretty straightforward idea. Now let's explore a
few more properties. Let's say I want to take
the derivative with respect to x of-- let's use
the same A. Let's say I have some constant
times some function. Well, derivatives
work out quite well. You can actually take this
little scalar multiplier, this little constant, and
take it out of the derivative. This is going to be
equal to A. I didn't want to do that magenta color. It's going to be equal to A
times the derivative of f of x. Let me do that blue color. And the other way to denote
the derivative of f of x is to just say that
this is the same thing. This is equal to A times
this thing right over here is the exact same
thing as f prime of x. Now this might all look
like really fancy notation, but I think if I
gave you an example it might make some sense. So what about if I were to ask
you the derivative with respect to x of 2 times x
to the fifth power? Well, this property
that I just articulated says, well, this is going
to be the same thing as 2 times the derivative of x to the
fifth, 2 times the derivative with respect to x
of x to the fifth. Essentially, I could just
take this scalar multiplier and put it in front
of the derivative. So this right here,
this is the derivative with respect to x
of x to the fifth. And we know how to do
that using the power rule. This is going to be equal to
2 times-- let me write that. I want to keep it
consistent with the colors. This is going to be 2 times the
derivative of x to the fifth. Well, the power rule
tells us, n is 5. It's going to be 5x
to the 5 minus 1 or 5x to the fourth power. So it's going to be 5x to
the fourth power, which is going to be equal to 2
times 5 is 10, x to the fourth. So 2x to the fifth,
you can literally just say, OK, the power rule
tells me derivative of that is 5x to the fourth. 5 times 2 is 10. So that simplifies
our life a good bit. We can now, using the power
rule and this one property, take the derivative
anything that takes the form Ax
to the n power. Now let's think about
another very useful derivative property. And these don't just
apply to the power rule, they apply to any derivative. But they are especially
useful for the power rule because it allows us to
construct polynomials and take the derivatives of them. But if I were to
take the derivative of the sum of two functions--
so the derivative of, let's say one function
is f of x and then the other function is g of x. It's lucky for us
that this ends up being the same thing
as the derivative of f of x plus the
derivative of g of x. So this is the same
thing as f-- actually, let me use that derivative
operator just to make it clear. It's the same thing as the
derivative with respect to x of f of x plus the
derivative with respect to x of g of x. So we'll put f of
x right over here and put g of x right over there. And so with the
other notation, we can say this is going
to be the same thing. Derivative with respect to x
of f of x, we can write as f prime of x. And the derivative with
respect to x of g of x, we can write as g prime of x. Now, once again, this might
look like kind of fancy notation to you. But when you see an example,
it'll make it pretty clear. If I want to take the
derivative with respect to x of let's say x
to the third power plus x to the negative
4 power, this just tells us that the
derivative of the sum is just the sum of
the derivatives. So we can take the derivative of
this term using the power rule. So it's going to be 3x squared. And to that, we can add the
derivative of this thing right over here. So it's going to
be plus-- that's a different shade of blue--
and over here is negative 4. So it's plus negative 4 times
x to the negative 4 minus 1, or x to the negative 5 power. So we have-- and I could
just simplify a little bit. This is going to be
equal to 3x squared minus 4x to the negative 5. And so now we have all the
tools we need in our toolkit to essentially take the
derivative of any polynomial. So let's give ourselves
a little practice there. So let's say that I have--
and I'll do it in white. Let's say that f
of x is equal to 2x to the third power minus 7x
squared plus 3x minus 100. What is f prime of x? What is the derivative of f
with respect to x going to be? Well, we can use the
properties that we just said. The derivative of
this is just going to be 2 times the derivative
of x to the third. Derivative of x to the third
is going to be 3x squared, so it's just going to
be 2 times 3x squared. What's the derivative
of negative 7x squared going to be? Well, it's just going
to be negative 7 times the derivative of x
squared, which is 2x. What is the derivative
of 3x going to be? Well, it's just going to be
3 times the derivative of x, or 3 times the derivative
of x to the first. The derivative of x to
the first is just 1. So this is just going
to be plus 3 times-- we could say 1x to the
0-- but that's just 1. And then finally, what's
the derivative of a constant going to be? Let me do that in
a different color. What's the derivative of
a constant going to be? Well, we covered that at
the beginning of this video. The derivative of
any constant is just going to be 0, so plus 0. And so now we are
ready to simplify. The derivative of
f is going to be 2 times 3x squared
is just 6x squared. Negative 7 times 2x is
negative 14x plus 3. And we don't have to
write the 0 there. And we're done. We now have all the
properties in our tool belt to find the derivative
of any polynomial and actually things that
even go beyond polynomials.