00:00Hi guys, so we're going
00:01to look here at the
00:02intersection of three planes. Now,
00:05planes can intersect in a
00:07few different ways. This picture
00:10shows three planes intersecting at
00:12a point. This picture shows
00:15three planes intersecting at a
00:19line, and this is three
00:21planes that actually don't intersect
00:22at all. Now, this intersection
00:26of planes is
00:28very much related to and
00:29connected to the system of
00:32linear equations lessons that I've
00:35done in the past. So
00:36here, when I have this
00:38system of linear equations with
00:39three variables, what I have
00:41is a system or what
00:42I have is three planes.
00:44So the system of equations
00:45that is describing three different
00:48planes. So if you haven't
00:52looked at that lesson, or
00:53you're not familiar with that
00:54lesson, you have to go
00:55and do that first
00:56because I'm actually not going
00:58to show you the solutions
00:59of these by hand. That's
01:01done in the previous lesson.
01:03What I do want you
01:04to see though is that
01:05by solving this, if I
01:06can solve these, the system
01:09of equations and find one
01:12solution, so a value of
01:14x, y and z, that
01:15satisfies the three of them.
01:16There's only one solution. Then
01:18I have this, I have
01:19this one unique solution. If
01:23I solve it and there's
01:24infinite and infinite number of
01:26solutions. I have this situation
01:28where they meet at a
01:29line. So this is a
01:30bit like, I think I've
01:32given the analogy of a
01:33book. Imagine you open up
01:34pages of a book where
01:36they all meet along the
01:37line there. And then a
01:38situation like this where there
01:41is no solutions. There's lots
01:43of different kind of situations
01:44where planes can have no
01:47solutions. And like imagine three
01:50planes are parallel. There's going
01:52and we know solutions. Or
01:53even two planes are parallel
01:54and one of them is
01:57not parallel. And even none
02:00of them are parallel. You
02:00could like this situation, neither
02:02of them are parallel, but
02:04the line this line here
02:06is actually parallel to say,
02:10I don't know, this plane
02:11here, something like that. So
02:13if this line is parallel
02:14to this plane, they'll never,
02:16they won't all intersect together.
02:20Okay, so let me just
02:21show you those three examples
02:23that I'm talking about in
02:24geodhya. So this is the,
02:27this is the first one.
02:29So here are three planes.
02:30They're all going to meet
02:31at a point. Now I
02:32actually have the point is
02:33this is the example I'm
02:34about to do for you.
02:35And the answer is one
02:36negative one three. So they
02:38meet at that point. So
02:39you can see it's on
02:40this plane. It's on this
02:42plane. And it's on this
02:44plane. So the point is
02:45on all three planes. That
02:47is the point of intersection
02:48three planes and when you
02:49solve this, which you're going
02:51to do in a second,
02:52you will get an x,
02:53y in a z, and
02:55that is the point. When
02:58planes meet at a line,
03:00it looks something like this.
03:02These are the next three
03:03I've drawn. These three. So
03:09you can see the meeting,
03:11they all meet on that
03:12line there.
03:16It's a bit like when
03:17two planes meet but three
03:19planes can also meet a
03:20line like that. Again, you
03:21might see my analogy of
03:23the book pages of a
03:24book. Although, yeah, this doesn't
03:28have to actually look exactly
03:30like a book in this
03:31situation. But as you know,
03:33those planes go on forever.
03:34So they're all meeting at
03:36a line. And then finally,
03:40finally, let me just change
03:44one of these planes. Imagine
03:46I make this nine, then
03:49let's make it a bigger
03:50number like, I don't know,
03:52let's make it thirty. Then
03:55what's happened here is these
03:57planes are not actually going
04:00to, these planes are not
04:02actually going to meet anywhere.
04:06Now this plane is going
04:07to be this plane, this
04:08plane is going to be
04:09this plane, but they're not
04:10all going to meet together
04:11so there's no interesting
04:12section of three planes and
04:14that's the situation that we
04:15have no solutions. So, as
04:21I said, I'm not going
04:22to do this without a
04:24calculator. I've done this in
04:25a previous lesson. Make sure
04:27you know how to do
04:27it. You need to be
04:28able to solve this system
04:30of equation without a calculator.
04:32I'm going to do it
04:33with a calculator because I'm
04:35lazy. Menu at calculator.
04:40menu algebra solve system of
04:45linear equations. Now I want
04:46three automatically gives me the
04:49z and then I just
04:51type them in x plus
04:52y plus z equals three
04:55x minus y plus z
04:59equals five and x plus
05:03y plus two z equals
05:07six press and
05:08and there I have my
05:10point. 1, negative 1 and
05:123. So when he says
05:15the point of intersection, the
05:16point of intersection is, and
05:25again guys by hand, that
05:27takes a bit of time
05:28to do, but it's 1,
05:30negative 1, 3, 1, negative
05:331, 3. Okay, next question.
05:36find the vector equation of
05:39the line of intersection of
05:40the three planes. Same thing.
05:43Make sure you know how
05:44to do this without a
05:47calculator. Go and check my
05:48lessons on solving systems of
05:51linear equations. But this is
05:52going to have an infinite
05:54number of solutions. So we're
05:56going to use Gaussian elimination.
05:58And you're going to get
05:59all zeros on the bottom.
06:01And then you have to
06:03let z equal t
06:04and you bring in the
06:08parameter. But again, I'm going
06:10to do it by using
06:14my calculator. So I do
06:16menu algebra solve system of
06:19linear equations, three equations. And
06:23then again, just type it
06:24in two x minus seven
06:28y plus five z equals
06:32one.
06:336x plus 3y minus z
06:43equals negative 1, negative 14x
06:49minus 23y plus 13z equals
06:575 press enter and I
07:00get this.
07:01So what is this? Remember,
07:06this is their way of,
07:07this is their way of
07:07bringing the parameters. Their parameter
07:09is C1, but that's what
07:13we're gonna call T. So
07:15what we have is x
07:17equals x equals negative of
07:2112 minus a sixth T.
07:25So it's minus 12 minus
07:29a sixth T.
07:29sixth t, that's this negative
07:3312th, first minus t over
07:36six or sixth t. Then
07:38the y value is minus
07:39a sixth plus two thirds
07:44t. So y is equal
07:47to minus a sixth plus
07:51two thirds t and finally
07:57Okay, z is just going
07:59to equal t then. So
08:01z equals t. And we
08:04can write this. They want,
08:05they say, find the vector
08:06equation of the line. So
08:08we want to write this
08:09in this form or equals.
08:12So I have negative 12,
08:17negative a sixth, and zero
08:21plus
08:25T times negative of sixth,
08:32two thirds T, sorry, just
08:36two thirds because the T's
08:37here, and then one. And
08:40this, when we draw this,
08:43this is the equation of
08:45the line that passes through
08:48them. Now let's actually draw
08:51the, well I won't do
08:53it,
08:53But if I was to
08:55draw that line in geodebra,
08:58now I have changed this,
08:59it was, equals five. So
09:05this is equal to five.
09:11So that's the line. So
09:12if I was to draw
09:12that line on geodebra, it
09:15would give me that line
09:17where they meet along there.
09:19Okay, that's it.
09:21That lesson was quite short.
09:24As I said three times
09:25now because I'm not I
09:27didn't do it by hand,
09:28but you make sure you
09:29know how to do it
09:30by hand. Go back and
09:31watch my system of linear
09:33equations lesson where I actually
09:36talk. I actually mentioned I
09:38say when we do vectors
09:39you'll understand what this means.
09:41So now you understand what
09:42it's all about. Okay, see
09:45you in the next video.