# Homework Help: Additive identity over linear transformation

1. Nov 21, 2011

### autre

Given vector spaces V, W over a field, and linear transformation $T:V\rightarrow W$, prove $T(0_{v})=0_{w}$ where 0_v and 0_w are additive identities of V and W.

I'm trying to use the definition of additive identity. So, $\forall\vec{v}\in V,\vec{v}+0=\vec{v+0=0}$. Where do I go from here?

2. Nov 22, 2011

### CompuChip

I don't really see why v + 0 = 0... unless v = 0.
However, you may want to look at v + (-v) = 0.

3. Nov 22, 2011

### autre

Woops, I meant $\forall\vec{v}\in V,\vec{v}+0=\vec{0+v=\vec{v}}.$ Not the additive inverse.

4. Nov 22, 2011

### CompuChip

Yep, I guess you can use that as well.
You will still need the additive inverse at some point though (if not in V, then at least in W to show that T(v) - T(v) = 0W).

5. Nov 22, 2011

### Theorem.

Here's a hint: Consider $T(0_v+0_v)$.

6. Nov 22, 2011

### autre

Would this line of reasoning work?

$T(0_{v})=T(0_{v}+0_{v})=T(0_{v}+\vec{v}-\vec{v}+0_{v})=T(0_{v}+\vec{v})-T(0_{v}+\vec{v})=T(\vec{v})-T(\vec{v})=\vec{w}-\vec{w}=0_{w}.$

7. Nov 22, 2011

### Theorem.

That looks good to me! In fact you can make it even simpler If you want by noting $T(0_v)=T(0_v+0_v)=T(0_v) + T(0_v)$ which is only possible if $T(0_v)=?$

8. Nov 22, 2011

### autre

$T(0_{v})=0_{w}$, or the additive identity in W?

9. Nov 22, 2011

### Theorem.

Exactly , it must be the zero of W.

10. Nov 22, 2011

### Fredrik

Staff Emeritus
It's correct, but you're using the following two results without proving them
• T(-x) is the additive inverse of T(x) for all x.
• -0=0
The first one is no more obvious than the statement you're trying to prove.

If you're going to use these results, there's no need to involve a new vector v. You could just write $$T(0)=T(0+0)=T(0+(-0))=T(0)+T(-0)=T(0)-T(0)=0.$$ I prefer the method Theorem suggested.

11. Nov 23, 2011

### CompuChip

An alternative way to write it down is:
T(0) = T(0 + 0) = T(0) + T(0) because of linearity.
Now add the additive inverse of T(0) (whatever it is, but there must be one) to both sides, and you get
T(0) + (-T(0)) = T(0) + T(0) + (-T(0))
0 = T(0)