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I don't know how can I to prove the following (this isn't homework, i'm just flipping through a book):

if a ∈ F, v ∈ V, and av=0

then a=0 or v=0

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That's nice. But this isn't twitter or facebook, so you don't have to update us on your status. (That is: you have a question? Please ask; say what you have done, where you are confused, or what you have tried. And if it is a homework assignment, please use the [homework] tag) –  Arturo Magidin Mar 12 '11 at 4:39
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And, again, and more directly: What have you tried? Where are you confused? And if this is homework, please add the homework tag. –  Arturo Magidin Mar 12 '11 at 4:46
    
Assume $av = 0$. If $a = 0$ you're done; otherwise, by the field axioms there is $b \in F$ with $ba = 1$; then the vector space axioms imply that $v = 1v = (ba)v = b(av) = b(0) = 0$ and you're done. [The fact that $b0 = 0$ may itself require proof.] –  anon Mar 12 '11 at 4:48
    
anon, thank you wish i could give you rep for your reply. from your answer i was able to see how to begin proving such problems –  Krolique Mar 14 '11 at 2:40
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1 Answer

up vote 1 down vote accepted

If it hasn't been proven already, try proving the preliminary facts that $0_Fv=0_V$ and $a0_V=0_V$ for $a\in F$ and $v\in V$. This follows by using that fact that $0=0+0$ (for both the zero element of $F$ and of $V$), and using distributivity.

If $a=0$, you are done. So suppose $a\neq 0$, and hence has a multiplicative inverse. Try multiplying both sides of your equation by the inverse of $a$, and recall the axiom that $1v=v$ where $1$ is the multiplicative identity of $F$, to get your conclusion.

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Thank you yunone ) I wasn't too sure how to tackle this –  Krolique Mar 14 '11 at 2:39
    
@Krolique Sure, I'm glad it helped. –  yunone Mar 14 '11 at 3:10
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