Help with a proof involving the span of a subset

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SUMMARY

The discussion focuses on proving that for a subset S of a vector space V and an element v in V, the equality span(S) = span(S ∪ {v}) holds if and only if v is an element of span(S). The proof involves two main steps: first, demonstrating that if v is in span(S), then span(S) equals span(S ∪ {v}); second, proving that if span(S ∪ {v}) equals span(S), then v must be in span(S). The definition of span as the space of all linear combinations of elements in a set is crucial to the argument.

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Homework Statement


Let S be a subset of a vector space V, and let v be an element of V. Show that span(S) = span(S U {v}) if and only if v is an element of span(S)


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The Attempt at a Solution



I'm honestly not sure how to get started, I've spent time looking through the text but it hasn't helped.
 
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You have to do two things.

1. Suppose that v is in span(S), show that span(S)=span(S u {v}).

2. Show that if span(S u {v}) = span(S) then v is in span(S).

Let's look at 2 and break down what you know.

Definition: span(X) is the space of all linear combinations of objects in X.

You need to show v is in span(S).

You know that v is in span(S u {v}) as it is a linear combination of elements of S U {v} trivially.

You know that span(S u {v}) = span(S).

Hence...

Now try 1.
 
Thanks for your help, it allowed me to finish the rest of the proof.
 

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