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To this point we’ve only looked at the two partial
derivatives 
and 
. Recall that these derivatives represent the
rate of change of f as we vary x (holding y fixed) and as we vary y
(holding x fixed) respectively. We now need to discuss how to find the rate
of change of f if we allow both x and y to change simultaneously.
The problem here is that there are many ways to allow both x and y to change. For instance
one could be changing faster than the other and then there is also the issue of
whether or not each is increasing or decreasing. So, before we get into finding the rate of
change we need to get a couple of preliminary ideas taken care of first. The main idea that we need to look at is just
how are we going to define the changing of x
and/or y.




Let’s start off by supposing that we wanted the rate of
change of f at a particular point,
say 
. Let’s also suppose that both x and y are increasing and that, in this case, x is increasing twice as fast as y is increasing. So, as y increases one unit of measure x will increase two units of
measure.
To help us see how we’re going to define this change let’s
suppose that a particle is sitting at 
and the particle will move in the direction
given by the changing x and y.
Therefore, the particle will move off in a direction of increasing x and y and the x coordinate of
the point will increase twice as fast as the y coordinate. Now that we’re
thinking of this changing x and y as a direction of movement we can get
a way of defining the change. We know
from Calculus II that vectors can be used to define a direction and so the
particle, at this point, can be said to be moving in the direction,


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Since this vector can be used to define how a particle at a
point is changing we can also use it describe how x and/or y is changing at
a point. For our example we will say
that we want the rate of change of f
in the direction of 
. In this way we will know that x is increasing twice as fast as y is.
There is still a small problem with this however. There are many vectors that point in the same
direction. For instance all of the following
vectors point in the same direction as 
.
We need a way to consistently find the rate of change of a
function in a given direction. We will
do this by insisting that the vector that defines the direction of change be a
unit vector. Recall that a unit vector
is a vector with length, or magnitude, of 1.
This means that for the example that we started off thinking about we
would want to use
since this is the unit vector that points in the direction
of change.
For reference purposes recall that the magnitude or length
of the vector 
is given by,
For two dimensional vectors we drop the c from the formula.
Sometimes we will give the direction of changing x and y as an angle. For instance,
we may say that we want the rate of change of f in the direction of 
. The unit vector that points in this direction
is given by,
Okay, now that we know how to define the direction of
changing x and y its time to start talking about finding the rate of change of f in this direction. Let’s start off with the official definition.
Definition
So, the definition of the directional derivative is very
similar the definition of partial derivatives.
However, in practice this can be a very difficult limit to compute so we
need an easier way of taking directional derivatives. It’s actually fairly simple to derive an equivalent
formula for taking directional derivatives.
To see how we can do this let’s define a new function of a
single variable,
where 
,

,
a, and b are some fixed numbers.
Note that this really is a function of a single variable now since z is the only letter that is not
representing a fixed number.
Then by the definition of the derivative for functions of a
single variable we have,
and the derivative at 
is given by,
If we now substitute in for 
we get,
So, it looks like we have the following relationship.
Now, let’s look at this from another perspective. Let’s rewrite 
as follows,
We can now use the chain rule from the previous section to
compute,
So, from the chain rule we get the following relationship.
If we now take 
we will get that 
and 
(from how we defined x and y above) and plug
these into (2)
we get,
Now, simply equate (1) and
(3)
to get that,
If we now go back to allowing x and y to be any number
we get the following formula for computing directional derivatives.
This is much simpler than the limit definition. Also note that this definition assumed that
we were working with functions of two variables. There are similar formulas that can be
derived by the same type of argument for functions with more than two variables. For instance, the directional derivative of 
in the direction of the unit vector 
is given by,
Let’s work a couple of examples.