In this section we want to revisit tangent planes only this
time we’ll look at them in light of the gradient vector. In the process we will also take a look at a
normal line to a surface.
Let’s first recall the equation of a plane that contains the
point with normal vector is given by,
When we introduced the gradient vector in the section on directional derivatives we gave
the following fact.
Fact
Actually, all we need here is the last part of this
fact. This says that the gradient vector
is always orthogonal, or normal, to
the surface at a point.
Also recall that the gradient vector is,
So, the tangent plane to the surface given by at has the equation,
This is a much more general form of the equation of a
tangent plane than the one that we derived in the previous section.
Note however, that we can also get the equation from the
previous section using this more general formula. To see this let’s start with the equation and we want to find the tangent plane to the
surface given by at the point where . In order to use the formula above we need to
have all the variables on one side. This
is easy enough to do. All we need to do
is subtract a z from both sides to
get,
Now, if we define a new function
we can see that the
surface given by is identical to the surface given by and this new equivalent equation is in the
correct form for the equation of the tangent plane that we derived in this
section.
So, the first thing that we need to do is find the gradient
vector for F.
Notice that
The equation of the tangent plane is then,
Or, upon solving for z,
we get,
which is identical to the equation that we derived in the
previous section.
We can get another nice piece of information out of the
gradient vector as well. We might on
occasion want a line that is orthogonal to a surface at a point, sometimes
called the normal line. This is easy enough to get if we recall that
the equation of a line only requires that we have
a point and a parallel vector. Since we
want a line that is at the point we know that this point must also be on the
line and we know that is a vector that is normal to the surface and
hence will be parallel to the line.
Therefore the equation of the normal line is,
Example 1 Find
the tangent plane and normal line to at the point .
Solution
For this case the function that we’re going to be working
with is,
and note that we don’t have to have a zero on one side of
the equal sign. All that we need is a
constant. To finish this problem out
we simply need the gradient evaluated at the point.
The tangent plane is then,
The normal line is,
