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(3) Let V denote a vector space over the field F and let v,..., Un E V. (a) Show that span(vn, 2,. , Un) (b) Show that span (
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Answer #1

(a) . Suppose x € span ( v1 , v2 , v3,....,vn) .

Then there exists scalars c1 , c2 , c3 , ...., cn € F such that

x = c1v1 + c2v2 + .....+ cnvn

Now , c1v1 € span (v1) ; c2v2 € span (v2) ,......., cnvn € span(vn)

\Rightarrow c1v1 + c2v2 + .....+ cnvn € span (v1) + span(v2) +....+ span(vn)

\Rightarrow x € span(v1) + span (v2) +....+span(vn)

So x € span ( v1 , v2 , ...., vn) \Rightarrow x€ span (v1) + span (v2) +....+span(vn)

Hence ,

span (v1 , v2,....,vn) \subset span(v1) + span(v2) +....+ span(vn) ..........(1)

Conversely suppose , y € span(v1) + span(v2) +....+ span(vn)

\Rightarrow there exist d1 , d2 , ....dn € F such that y = d1 v1 + d2 v2 + ...+ dn vn

\Rightarrow y € span (v1 , v2,....,vn)

So y € span(v1) + span(v2) +....+ span(vn) \Rightarrow y€ span (v1 , v2,....,vn)

Hence ,

span (v1) +span(v2) +....+span(vn) \subset span ( v1 , , v2,....,vn) ..........(2)

From (1) and (2) we get that ,

span ( v1 , , v2,....,vn) =  span (v1) +span(v2) +....+span(vn) .

(b) we have already prove that span ( v1 , , v2,....,vn) =  span (v1) +span(v2) +....+span(vn) . To prove that span ( v1 , , v2,....,vn) =  span (v1) \oplusspan(v2) \oplus...\oplusspan(vn) we only need to prove that ,

span (v1) \cap span (v2) \cap .....\cap span (vn) = {0}

Let x € span (v1) \cap span (v2) \cap .....\cap span (vn)  

\Leftrightarrow x € span (v1) , x € span (v2) ,.....,x€ span (vn)

\Leftrightarrow x = a1v1 , x=a2v2 , .....,x = anvn.

\Leftrightarrow a1v1 = a2v2 = ......= anvn

\Rightarrow( v1 , v2 , .....,vn) is linearly dependent , a contradiction .

Hence , span (v1) \cap span(v2) \cap.....\cap span (vn) ={0}

.

If you have any doubt or need more clarification at any step please comment.

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