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# Rowspace solution to Ax = b example

Video transcript

I've got this 2 by 2 matrix A
here, and I've got this other member of R2, this vector b. Let's figure out all of the
interesting things that we can figure out about this matrix
and this vector. So the first thing of interest,
and this is all going to essentially help us
visualize what we learned in the last video, is the
null space of A. To figure out the null space
of A, we know that the null space of A is equal to the null
space of the reduced row echelon form of A. So let's just find out the
reduced row echelon form of A. Let's say we leave the
first row the same, we get 3 minus 2. And let's replace the second row
with the second row, minus 2 times the first, so 6,
minus 2 times 3 is 0. And minus 4, minus 2
times minus 1 is 0. And then let's replace the first
row with the first row divided by 3. So then that becomes 1 minus
2/3, and the second row is still 0. So that's the reduced row
echelon form of A. And we want to find
its null space. So we want to find all of
the vectors that we can multiply it by. So this is the vector x1,
x2, that is equal to the 0 vector in R2. So the second row gives us no
information, 0 times x1, plus 0 times x2 is equal to 0. No information there. So our only constraint
is the first row. 1 times x1-- let me write it
here-- so 1 times x1, plus-- let me write it this way--
minus 2/3 times x2 is equal to that 0. Is equal to that
0 right there. Or we could write that x1
is equal to 2/3 x2. So if we wanted to write the
null space of A-- actually, before I write that, just to
simplify things and just to show you that x2 isn't some
special number, let's just say that x2 is equal to t, where
t is some real number. And then we would have x1
is equal to 2/3 times t. So the null space of our matrix,
the null space of A, is going to be equal to the set
of all x1, x2's, that are equal to some real number
t times the vector. x2 is equal to t times 1, and
x1 is equal to 2/3 t. So 2/3, 1. Just like that. That is our null space. All of the multiples of
the vector 2/3, 1. And actually just to make this
a little bit simpler, we can pick c, let's pick t,
sorry, let's pick t to be equal to 3c. So then what do you have? If this is equal to 3c, if you
multiply the 3 times these, we could rewrite this as being
equal to the x1's, x2's that are equal to the scalar c, where
that is some other real number, times 3 times 2/3
is 2, 3 times 1 is 3. And the whole reason why I did
that is just to write the null space with a slightly simpler
basis vector. The one that didn't have
any fractions in it. Or we could also write that
the null space is equal to the span. The span of the vector 2, 3. All of these statements
are equivalent. So we figured out
the null space. Maybe another interesting thing,
especially if we're going to make this relate to
what we did in the last video, is find a solution set to the
equation Ax is equal to b. To do that, we just set up
an augmented matrix. So we set up an augmented
matrix, 3 minus 2, 6 minus 4, and we augment it
with b, 9, 18. And then we put the left-hand
side in reduced row echelon form, and we get-- let's keep
the first row the same-- we get 3 minus 2, and then 9. Let's replace the second row
with the second row minus 2 times the first. So we get 6
minus 2 times 3, is 0, minus 4 minus 2 times minus 2,
that's 0 as well. Minus 4 plus 4. 18 minus 2 times 9, that's 18
minus 18, that's also 0. Almost there. Now let's just replace the first
row with the first row divided by 3. So the second row is going
to stay the same. And the first row is going to
be 1 minus 2/3, and then we get a 3 there. So if we wanted to rewrite this
is as an equation, we would write that the matrix 1
minus 2/3, 0, 0, times the vector that's in our solution
set, x1, x2, is equal to the vector 3, 0. Or another way to say it, the
second row gives us no real constraint, so we don't have
to pay attention to it. The first row tells us that
x1 minus 2/3, times x2 is equal to 3. Or, that x1 is equal
to 3, plus 2/3 x2. Let's do the same
exercise here. If we say that x2 is equal
to t, then x1 is equal to 3 plus 2/3t. Or we could say that the
solution set to Ax is equal to b is equal to the set of all x1,
x2's that are equal to-- let's see, x1 is equal to 3 plus
2/3, so plus t times 2/3. Let's write it like that. So plus right there. And x2 is just equal to t, so
let's just go 1 times t plus, well let's just say 0. So that's our solution
set right there. And you might immediately
recognize that it's some particular solution plus some
scaled up version of our null space right there. That's our solution set. I could do that same
substitution that we did over here. I could say that let's replace
t with 3c, and so we could rewrite this as being equal to--
let's rewrite it neatly-- x1, x2 is equal to the vector--
I'm going to run out of space-- 3, 0, plus c, times--
let me scroll over a little bit-- plus c, times the
vector, 3 times 2/3 is 2, 3 times 1 is 3. So it's equal to the vector 3, 0
plus some member of our null space, right? The null space was the span
of 2, 3, or all the scalar multiples of our null space. And this is where c is
any real number. So that's our solution set. That's our null space. Now, one other thing that might
be of interest, just because it's the orthogonal
complement to the null space, and this relates to what we were
doing in the last video, is what the row space of A is. So the row space of
A is just the column space of A transpose. This is the row space. This is the row space of A. And what is this equal to? This is equal to the span
of the row vectors of A. So we have 3 minus 2, and
we have 6 minus 4. But this guy is just 2 times
that vector right there, so we can just ignore it. So it's just the span of
the vector 3 minus 2. Now let's graph all of these. So we have the row space, we
have the solution set and we have the null space. Let's see if we can graph it. We can make a reasonable graph
of this thing right here. So I have my vertical
axis right there. I have my horizontal
axis right there. Now what does the null
space look like? It's all the multiples
of 2, 3. So if I go out 1, 2 and then I
go up 1, 2, 3, the vector 2, 3 looks like this in the
standard position. And the null space is all of
the multiples of this. So, all the multiples of that. So if you do all the multiples
of that, you get a line that looks something like this. If you do all the multiples of
that vector, you get a bunch of vectors that point to
every point on this green line right there. So that right there is
the null space of A. Now what is the solution set? The solution set is
the vector 3, 0. So it is the vector
3, 0, so 1, 2, 3. So it is the vector 3 , 0, plus members of the null space. So if you take any of these --
let's say if you add 2, 3-- 2, 3 would look like this. But you're adding any
multiple of 2, 3. So when you add all the
different multiples of 2, 3, you get this line right here. It's shifted, essentially,
by that vector 3, 0. So this right here is
the solution set. So solution to Ax
is equal to b. That's the solution
set right there. Now, what's our row space? The row space is all of
the multiples of the vector 3 minus 2. So what does 3 minus
2 look like? The vector 3 minus 2, we go 1,
2, 3, and then we go down 2. So you go 1, 2, and it's going
to look like this. That's going to look something
like this, and then if you were to graph it and you can
see, well, it's going to look something like-- I want to do it
relatively neatly-- so it's going to look something
like this. Let me do it in another
color actually. It's going to look like
that right-- I can't do it at all straight. Oh, this is horrible. Something about the bottom right
hand side of my-- that's a pretty decent attempt. So that right there
is my row space. Because if you have 3 minus 2
you get roughly to that point, and then you want all
of the multiples of that vector, right? 3 minus 2, if you multiplied it
by minus 1, you would get the vector minus 3, 2, so
it would look like this. It would be minus 3, 2. It would look like this, 1, 2,
3 and then up 2, it would be like that, so all of the scalar multiples of that as well. That is our column space,. And notice-- I'm sorry that is
our row space-- and notice that our row space is orthogonal
to our null space. Now this is a nice visual
representation of everything we can do essentially with this
matrix right here, which is our matrix A. But in the last video, we had
a very interesting result. We had a very interesting
result. We said that if we have some
b-- let me do it in a new color-- we found out that if we
had some b that is a member of the column space of A, then
the solution with the smallest length, or we can say the
smallest, or the shortest solution to Ax is equal to b is
a unique member of the row space of A. Now let's see, this is our big
takeaway from the last video. And it might have seemed
a little hard to visualize before. But now that we have graphed
it all out, I think we can visualize it. This blue line right here
is a solution set. The row space right here is this
line that's perpendicular to the solution set. But notice, one of the vectors
on it is both pointing to something, pointing to a
position on the solution set, and it is in my row space. This vector right here, that
I'm doing in a bold green, That vector right there, we
could call that vector r, because that vector is both a
member of my row space, right? It points-- if you do it as a
position vector, it points to a point on the line that
represents our row space. All of the members of our
row space point to points on this line. But at the same time, it also
points to a point right there that is a member of
our solution set. And notice, it is the only
vector in our row space that points to a point that is a
member of our solution set. And if you just look at it
from a geometric point of view, all of the other solutions
point to all of the other points on that line. So, that's a solution right
there, this is a solution right there, that is a
solution right there. All of the vectors that point
to things on this solution line right there, there's
all solutions. But the very shortest one
is this green one. This green one is essentially
orthogonal to it because it's a member of the row space. And this essentially has the
same slope as our null space. It's orthogonal, so it's kind of
the shortest path distance to getting to our
solution set. And for this exact example, we
can actually figure out what this shortest vector r is. We've got the vector 3,
0, over here, right? The vector 3, 0 is this
vector right here. So if you take 3, 0 minus your
vector-- so let me say r. Let me write this way. Our vector r, the special
shortest solution, it's going to be a member of our
column space. Our column space is a
span of 3 minus 2. So it's going to be equal
to some scalar multiple of 3 minus 2. Now, we know that 3, 0 points
to another solution on our solution set. But we know that if we take the
difference of these two vectors-- if I take 3, 0 and
from that I subtract the vector r, what do I get? I get this vector right there. Let me do that in
another color. I get this pink vector
right there. That pink vector, it's not in a
standard position, but that pink vector is going to be a
member of our null space. That pink vector is going to be
a member of our null space. So if I take 3, 0. That, minus my r, so minus c,
times 3 minus 2, right? That's going to be this pink
vector right there. So if I were to take the dot
product, so this vector right here-- let me make this clear;
that vector is this pink vector right there-- that's
in my null space. I could shift it over. It's not in standard position
there, but [? I'll have to ?] do it in the standard position,
it points to something on my null
space line. So that's in my null space. So if I were to dot it with any
member of my row space, it's going to be equal to 0. The row space is the orthogonal
complement of the null space. So let me take the dot product
of that, with some member of my row space. Might as well just take the
basis vector for my row space. That's a member of
my row space. So if I dot it with 3-- sorry, I
don't want you to write that as a fraction-- 3 minus 2. If I dot it with 3 minus 2,
that will be equal to 0. So let's see if we
can solve for c. So we get, let's see if we do
this, we get this inside part, this pink vector right
there is going to be 3 minus 3c, right? And then you're going to
have 0 minus minus 2c, so that's just 2c. So this part right here
simplifies to that. I just perform the scalar
multiplication and then the subtraction. And then if we dot that with
my basis vector for my row space, 3 minus 2, this
should be equal to 0. So what do we get? We get 3 times 3, which is 9,
minus-- let me see, 3 times 3-- minus 3, or minus 3c times
minus 2-- let me write it this way-- plus-- let me
do it this way. This is probably the easiest
way to do it. Sorry, I was actually
doing it wrong. This is the first entry,
so you have 3 minus 3c, times 3, right? 3 minus 3c-- the first entry
times the first entry here-- plus 2c, times minus
2, is equal to 0. I was performing that dot
product in a strange way. I think I got a little caught
off guard by these two terms. But anyway, the first term times
the first term, plus the second term times
the second term. That's what our dot
product is. This isn't the matrix. This is just a first and a
second term right there. So then what do we have here? We have 3 times 3 is 9, minus
9c, minus 4c, is equal to 0. This is 9 minus 13c
is equal to 0. Or we get 13, let's see, 9
is equal to 13c, or c is equal to 9 over 13. So just like that we've solved
for our special vector r. We said, hey look, if you take
this 3, 0 vector and the difference between that and our
vector r, you're going to get some vector, right here,
that is in our null space. And if you take the dot product
of that with some member in our row space,
you're going to get 0. And the dot product is that
entry times that entry, plus that entry times that entry. And so you get c is equal
to 9 over 13. So our special vector r, the
unique shortest solution to the equation Ax is equal to 0,
is 9 over 13 times the vector, times our basis vector 3 minus
2, or we could write that as 27 over 13, right? And then we have 3 times--
and then 2 times 9 is minus 18 over 13. And that right there is our
shortest vector in our row space that satisfies
this equation. Let me write it better. It's the unique member of our
row space that also happens to be the shortest solution
to Ax is equal to b. So this is an example of what
we talked about in the last video, and hopefully you see
visually why this is so.