00:00
So now that we've learned
how to calculate the gradient
on a curve.
00:05
We have also moved that forward,
with the idea forward,
to calculating the gradient
at a particular point
on the curve.
00:12
So all we did remember was put x
values or later on you will see
even y values into your
gradient, into your dy by dx
to find a numerical gradient at
a particular point on the curve.
00:23
We are going to take this
idea a little bit further now.
00:27
And we are going to start to
look at gradients of tangents
and normals.
00:31
And we are also going to move
forward and actually try
and find equations of
tangents and normals.
00:36
Remember what I said that this
is quite a useful concept
specially when you are measuring
the decay
or growth of any substance.
00:43
Say perhaps in the field of
medicine, you can actually work
out with a gradient of
a particular point is.
00:49
And you can work out on
the equation of the tangent
or the equation that runs
next to that point
and extrapolate it to predict
further results.
00:57
Let's have a look at a curve.
01:00
So we've been dealing with these
types of curves previously.
01:04
So we've been looking
at a quadratic equation
where we are given the function
and we actually
find the gradient of that function.
01:12
However, when we start to talk
about tangents and normals,
say for example, you are asked
to find the gradient
at a particular point.
01:21
So let's just call this
point here point P.
01:23
You can find this point P
or you can find the exact gradient
of that point P
by substituting your x values
into there.
01:32
But another thing you can do
from here, is also find
the gradient of the tangent.
01:37
So the tangent runs right
next to this point here.
01:41
So it's almost like it touches
the point P
or just touches the one point
on the curve here.
01:47
So this is just one point.
01:50
And this here is the tangent.
01:54
So remember that tangent
is any line that just touches
the curve at one point.
02:01
So it may not look like
it does in my picture.
02:04
But it is just suppose to be
touching the point at one point.
02:08
And the amazing thing about this
point here, is that the gradient
of the curve and the gradient
of the tangent
are the same at this point.
02:15
So in this case you can say that
the gradient at P of curve
is the same as the gradient
of tangent at P.
02:29
I don't particularly have
to say that this point
for this gradient
of the tangent.
02:33
Because this is just
a straight line.
02:35
And if I know the gradient here,
I will know the gradient here
and I will know the gradient
here or at any point
of that line.
02:42
Because the gradient
of a line is constant,
whereas the gradient of a curve
is ever changing.
02:48
So in order to find the gradient
of a curve at a particular
point, we will have to
substitute your x
or y values into it.
02:55
However, if you are looking for
the gradient of a straight line,
you just need to find
the gradient at one point.
03:01
And so you'll have the gradient
of the entire line.
03:04
So let me just recap.
03:07
That if you want to know
the gradient of a tangent that
touches the curve at one point,
you can find the gradient of
the curve at that one point
and these share the same gradient
which makes a life
lot easier for us.
03:19
Another thing that could come
up in these types of cases
or you might be interested is
the gradient of the line
that runs perpendicular to this.
03:29
So what if we want to know
the gradient of this line here.
03:32
This line here meets
at 90 degrees.
03:36
It's perpendicular
to this tangent.
03:39
And we usually refer to
this line as the normal.
03:42
Now remember that the normal at
this point perhaps would have
the same gradient but it
isn't running along
or it isn't running along
the same.
03:54
It isn't touching that point
in the curve and running
along it.
03:57
So this gradient will be
different to the gradient
of the tangent.
04:01
And we will discuss
the combination of the two
shortly.
04:05
The normal is the line that goes
90 degrees to the tangent,
it meets at 90 degrees.
04:11
And this is a special kind
of line because it meets
the 90 degrees.
04:15
And they hold a special
relationship with their
gradients.
04:18
Now say that this line
or the tangent has gradient m1.
04:22
Let's just call this m1 for now.
04:24
And let's called the gradient
of the normal m2.
04:29
If you think about this,
if this gradient here,
if this gradient here is positive,
it's going up, you can almost
see that this gradient here is
going to be negative.
04:40
That's only true in this case.
04:43
Obviously your tangent could
have a negative gradient
and the normal could have
a positive gradient.
04:47
But it's quite important to
notice that there is a change in
sign that's fairly evident here.
04:52
So you can see that it's
either positive or negative.
04:55
And they share that
opposite sign relationship.
04:59
The definition for this is that
if two lines are perpendicular,
then m1 multiplied by m2 is
going to give you an answer of
minus 1.
05:10
And this is only true
for perpendicular lines.
05:17
And that's how we can link
tangents and normals together.
05:20
So if you remember this
little rule here on the side.
05:23
If we know the gradient m1,
we can find here work out
the gradient too, just by using
this relationship that the two
gradients should multiply to
give you an answer of minus 1.
05:37
Similarly, you can think
about two parallel lines.
05:41
So if I just do that here on
this side here, so if I have two
parallel lines here, it doesn't
matter what length they are.
05:47
If say this gradient
is called m1
and this gradient is called m2.
05:51
We know that if two lines are
parallel, they must have
the same gradient.
05:56
They don't at any point
meet or intersect.
05:58
So if m1 is equal to m2, then
these lines are parallel.
06:06
And these are some of the basics
that you may have already
covered in your previous
studies of maths.
06:12
But this can just
be a recap for us.
06:14
And it's important for
us to remember this.
06:16
Specially this little rule here.
06:20
Because you will see that we're
going to use a lot of this
as we find equations of tangents
and normals.
06:25
So let me just go over
this quickly.
06:27
The equation of the tangent
you can find by finding
the gradient of a point
on the curve here.
06:33
And that gradient is shared.
06:37
So it's exactly the same
gradient at the point P
as it is of the line here
or the tangent.
06:42
And if from here you want to
calculate the gradient of
the normal, you could use this
formula that m1 multiplied
by the m2 will give you
an answer of minus 1.
06:51
If in a scenario your
two lines are parallel.
06:55
So if they run parallel
to each other,
you know in that case
the gradients must be the same.