Question

For which scalars a, b, c, d do the solutions to the equation ax+by+cz=d form a subspace of R(3).

Answer 1

In \(ax + by + cz = d\), if x and y are any number in R(1), then z is constrained to
\[z=\frac{d-ax-by}{c}\]
The subspace S = { any (x, y, z)} is a plane which does not pass through (0,0,0) and is therefore not a subspace.

Answer 2

Actually, I failed to address all the trivial cases.
If a = b = c = 0, there is no solution. The empty set is not a subspace.
If one or two of a,b, or c equals 0, then you can rotate your axes such that the one that is not 0 is the constant for z, in which case the theorem holds.

Answer 3

so your saying that if atleast a, b, or c is 0 then the others can be non zero numbers and be a subspace of R(3)?

Answer 4

No, I'm saying that if d = 0 then the solution set (x, y, z) is a subspace of R(3) UNLESS all three are equal to 0.

Answer 5

ll three a, b, c****

Answer 6

and that would be the same thing if it was a, b, c to the equation ax+by=c form a subspace of R(2)... then c=0 in this case.

Answer 7

also, if d=0, then as long as atleast one of a, b, or c is non zero then it IS a subspace right?

Answer 8

actually the set u r given is a subspace if and only if d=0.

Answer 9

but why? and does it matter what a, b, and c are?

Answer 10

no.

Answer 11

u have
\[ \large D=\{(x,y,z):ax+by+cz=d\} \]

Answer 12

if d=0 then u have
\[ \large D=\{(x,y,z):ax+by+cz=0\}. \]
So let \(u=(x_0,y_0,z_0)\), \(v=(x_1,y_1,z_1)\) both in \(D\) and \(\alpha\in\mathbb{R}\) so
(i) \(u+v=(x_0+x_1,y_0+y_1,z_0+z_1)\) so
\[ \large a(x_0+x_1)+b(y_0+y_1)+c(z_0+z_1)= \]
\[ \large =(ax_0+by_0+cz_0)+(ax_1+by_1+cz_1)=0+0=0 \]

(ii) \(\alpha v=(\alpha x_1,\alpha y_1,\alpha z_1)\) so
\[ \large a(\alpha x_1)+b(\alpha y_1)+c(\alpha z_1)=\alpha(ax_1+by_1+cz_1=
\alpha0=0 \]
this proves that D is a subspace of R^3.

Answer 13

For the converse. Let's suppose that
\[ \large D=\{(x,y,z):ax+by+cz=d\} \]
is a subspace of R^3. then the zero vector MUST be in D so
\[ \large 0=a0+b0+c0=d \]

Answer 14

I do understand that for sure. but how did you know to make d=0, or how do you know it doesn't work for any non zero number?

Answer 15

ohhh okay.

Answer 16

therefore D is a subspace if and only if d=0

Answer 17

so that same idea applies if the question is ax+by=c, for scalars a, b, c ... subspace of R(2) right

Answer 18

yes

Answer 19

It seems like the book is trying to trick me... why give one question stating for ax+by=c, and then the question right after that be ax+by+cz=d, when c has to be 0 for the first and d has to be 0 for the second. Its the same question isnt it, or is there more to it that I am missing.

Answer 20

yes it is.

the book is trying to stress that for a subset of a given to be a subspace (i.e., a space on its own) the zero vector (which is unique) must also belong to the subset in question.

Answer 21

that's what u r supposed to get from this two exercises.

Answer 22

*given space*

Answer 23

i hope i was helpful
gotta go

Answer 24

it most definitely was... i hope to see you around again, you have been very helpful. thank you.