# Math Help - Determine which of the following are subspaces of R^3

1. ## Determine which of the following are subspaces of R^3

Could anyone explain why:

All vectors of the form (a, 0, 0) is closed under addition and multiplication,

(a, 0, 0) + (b, 0, 0) = (a + b, 0, 0)

k (a, 0, 0) = (ka, 0, 0)

While something like:

All vectors of the form (a, 1, 1)

Is not closed under addition or multiplication?

(a, 1, 1 ) + (b, 1, 1) = (a + b, 2 ,2)

k( a, 1, 1) = (ka, k, k)

I worked it out, but I don't know why this is not closed under addition or multiplication.

Any help is appreciated!

2. Originally Posted by DarK
Could anyone explain why:

All vectors of the form (a, 0, 0) is closed under addition and multiplication,

(a, 0, 0) + (b, 0, 0) = (a + b, 0, 0)

k (a, 0, 0) = (ka, 0, 0)

While something like:

All vectors of the form (a, 1, 1)

Is not closed under addition or multiplication?

(a, 1, 1 ) + (b, 1, 1) = (a + b, 2 ,2)

Look! Is the vector $(a+b,2,2)$ of the same form as $(a,1,1)$ ? Of course not since the later has 1 in its 2nd and 3rd coordinates!...and there you've shown why this set isn't closed under sum (and something very similar shows that it isn't closed under multiplicatiom, either).

Tonio

k( a, 1, 1) = (ka, k, k)

I worked it out, but I don't know why this is not closed under addition or multiplication.

Any help is appreciated!
.

3. What about the first coordinate a+b?

4. Originally Posted by DarK
What about the first coordinate a+b?

Who cares? In fact a+b is a general element, just as a, b or x, but who cares? The 2nd and 3rd coordinates decide the question.

Tonio