AASL 5.4 Tangents and normals
Learn AASL 5.4 Tangents and normals in this free IB Mathematics Analysis and Approaches (AA) video lesson for SL 5.4—Tangents and normal.
Learn AASL 5.4 Tangents and normals in this free IB Mathematics Analysis and Approaches (AA) video lesson for SL 5.4—Tangents and normal.
Learn AASL 5.4 Tangents and normals in this free IB Mathematics Analysis and Approaches (AA) video lesson for SL 5.4—Tangents and normal.
In this lesson, the process of finding the equations of the tangent and normal lines to a curve is explained, emphasizing the connection between tangents and derivatives. The tangent line touches the curve at a single point without crossing it, while the normal line is perpendicular to the tangent.
To find these equations, the following steps are outlined:
The final equations are presented in different forms for clarity, demonstrating the versatility of the approach.
00:00Hi guys. So in this
00:02lesson, what we're going to
00:03do is find the equation
00:04of the tangent and the
00:06normal to a curve. Now
00:08there's a strong connection between
00:10tangents and derivatives because the
00:14derivative actually gives you the
00:17gradient of the tangent of
00:19the curve. So it would
00:20be nice to and certainly
00:21very useful to be able
00:23to find the equation of
00:25a given tangent. Now what
00:29tangent is a straight line.
00:32The touches the curve, well,
00:35that just touches the curve
00:36at a particular point like
00:38that. So it's just touching
00:40it there. It doesn't cross
00:41it. It just touches it
00:44just once. That is the
00:47tangent. The normal, the normal
00:53to the curve. So that
00:54is the tangent to the
00:56curve
00:56of y equals f of
00:56x at this point. The
00:59normal to the curve y
01:01equals f of x at
01:02that same point is this
01:05line here. Let me try
01:07and draw it there. Now,
01:13what have I drawn? At
01:15what I've drawn, a perpendicular
01:18line. So the normal is
01:21perpendicular to the tangent.
01:24this is the normal. Okay,
01:31that's it. That's the tangent.
01:33That's the normal. Now we're
01:34going to find the equation
01:35of both. So to find
01:37the equation of a line,
01:39look, I've given this straight
01:40line equations here, section 2
01:43.1. This is the formula
01:44that I am going to
01:46focus on. This is y
01:47minus y minus y minus
01:47y minus m to x.
01:49M into x minus x,
01:50one. This is my favorite
01:51formula.
01:52as I've said before, you
01:53just find the gradient and
01:55a point and then you
01:56sub it into the formula.
01:58So that's what we're going
01:59to do down here. We
02:00need to find the gradient,
02:07but we need a point,
02:09we need a point, a
02:10gradient and the formula. So
02:12let me write those three
02:13things. We need a point,
02:17we need a gradient,
02:20and we need the formula.
02:25So let's start with, let's
02:28start with the point, the
02:30point. So it says find
02:34the equation of the tangent
02:35and the normal to the
02:37curve y equals x squared
02:38at x equals three. So
02:41the point we have, we
02:43have the x coordinate, but
02:46the y coordinate I don't
02:48have,
02:48But I can easily sub
02:49it in here. So when
02:51x is 3, y is
02:533 squared, which is 9.
02:56So let's just, let's actually
02:57show that working. So at
03:00x equals 3, at x
03:05equals 3, y equals 3
03:08squared, which is 9. Therefore,
03:11the point is 3 9.
03:16easy. Next thing we need
03:19is the gradient. Now the
03:21gradient, how do I find
03:24the gradient? Well there's the
03:27gradient formula if I'm dealing
03:28with straight lines but when
03:29I'm dealing with curves the
03:31way we find the gradient
03:32is the derivative. Remember that
03:35the derivative gives you the
03:37gradient. So whenever you see
03:38the word gradient or slope
03:41it usually means you
03:44you need to differentiate unless
03:47you're dealing with actual straight
03:49line geometry. But when you
03:52see that we're gradient, think
03:54differentiate. So I'm going to
03:55differentiate this. So define the
03:57gradient, I need to differentiate
03:59y equals x squared. The
04:03derivative dy dx equals 2x.
04:08That's a redefrencie, 2 times
04:10x. Let me know in
04:11the power of the derivative.
04:12is 2x and the derivative
04:14at, I'm going to write
04:18it like this, 2idx at
04:20x equals 3 equals 2
04:23times 3, which is 6.
04:28So the gradient of this
04:30function when x equals 3
04:32equals 6. Now, be careful.
04:36That is the gradient of
04:38the tangent. So I'm going
04:40to write here.
04:40M, and a little capital
04:43T. If I can say
04:44that in a little capital
04:45T, the gradient of the
04:47tangent equals 6. If the
04:51gradient of the tangent equals
04:536, the gradient of the
04:55normal M with a little
04:56capital N equals negative 1
05:00over 6. I have definitely
05:02gone over that in previous
05:03lessons. The perpendicular line, you
05:06flip it and change the
05:07sign. I'll do one more
05:08For example, if your MT
05:11was 3 quarters, then your
05:15M normal is negative 4
05:18over 3. That is essential
05:23that you know how to
05:25do that. So now I
05:26have the gradient for the
05:27tangent and I have the
05:28gradient for the normal. Now,
05:31next thing I need to
05:32do is sub it into
05:34the formula.
05:36Normally, no point intended, you
05:40would be asked to find
05:42one of them, find the
05:43equation of the tangent or
05:45find the equation of the
05:46norm. It's rare that they'd
05:47actually say find the equation
05:48of the tangent and the
05:49equation of the norm. But
05:50I just wanted to show
05:51you how to do both.
05:53And both, you do both
05:55the exact same, you get
05:56the point, you get the
05:57gradient, and you sub it
05:58into the formula. The only
06:00difference is for the tangent,
06:01you use this gradient, and
06:03for the normal, you use
06:04this
06:04the negative reciprocal. So the
06:08formula is y minus y1
06:11equals m into x minus
06:14x1 straight from the formula
06:16booklet. And this is why
06:18I like it. You're subbing
06:20in your point. y minus,
06:24let's actually write over here.
06:27x1 y1. So it's y
06:32minus 9.
06:33equals M. So for the
06:37tangent, it's 6 into x
06:43minus 3. I'm just going
06:46to multiply this out. Give
06:47me y minus 9 equals
06:506x minus 18. And if
06:53I put it in this
06:54form y equals 6x plus,
06:59I'm sorry, minus
07:019, so it's negative 18
07:02plus 9. So this is
07:04the gradient of the tangent.
07:09Maybe I should have given
07:13it a little title here.
07:16So, not that. I'm going
07:19to make this tangent. Then
07:28this will be
07:29the normal. So the norm
07:33is the same thing. It's
07:35y minus y1, same point,
07:39equals m. This is the
07:40only thing I'm changing. Instead
07:42of 6, it's negative 1
07:44over 6, m into x
07:47minus x1. Do I want
07:53to leave it like this?
07:55Probably not. Let's put this
07:56in the other
07:57So I'm going to multiply
07:58across by 6. So this
07:59is 6y minus 54 equals
08:06negative x minus 3. 6y
08:11minus 54 equals negative x
08:16plus 3 because I'm doing
08:18minus x and then minus
08:19times minus this plus and
08:21I'm going to bring or
08:22add this x over here.
08:24So this is x.
08:25plus 6y, x plus 6y,
08:32and then minus 57 equals
08:370. So look, it didn't
08:41ask me to put it
08:42in any particular form. The
08:45tangent I put in the
08:46form ycosm x plus c,
08:47and the normal I put
08:48it in the form a
08:49x plus b y plus
08:50c equals 0 only to
08:52show you the
08:53two different forms. So just
08:57to recap. What is a
09:01tangent? It is a straight
09:02line that touches the curve
09:05at one particular point. And
09:07when I say touch, I
09:08mean it just touches and
09:11moves away. It doesn't go
09:12through it or anything like
09:12that. The normal is a
09:15straight line that is perpendicular
09:16to the tangent. How do
09:19we get the equations of
09:20these?
09:21Well, you do these three
09:24steps for the equation of
09:25the line. You need a
09:27point, a gradient and a
09:28formula. The point you find
09:30from, well, it'll give you
09:32part of the point, if
09:33not all of the point.
09:34The gradient you find from
09:36differentiating, that's the important bit,
09:38and then you sub into
09:39the formula, making sure to
09:42remember if it's the normal
09:43that you do this flip
09:45and change the sign.