# Linear Algebra: Direct sum proof

by loesch.19
Tags: direct sum, linear algebra
 P: 3 Let U and V be subspaces of a vector space W. If W=U $$\oplus$$ V, show U $$\bigcap$$ V={0}. I'm a bit lost on this one... as I thought this was essentially the definition of direct sum. I'm unsure where to start. Any help would be great!
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,755 What's the definition of a direct sum you're using?
P: 3
 Quote by vela What's the definition of a direct sum you're using?
I wasn't sure if that was necessary info or not... looks like I was wrong :)

If U and V are subspaces of vector space W, and each w in W can be written uniquely as a sum u+v where u is in U and v is in V then W is a direct sum of U and V.

 Emeritus Sci Advisor PF Gold P: 16,091 Linear Algebra: Direct sum proof So were you able to do the problem?
P: 3
 Quote by Hurkyl So were you able to do the problem?
No... I still need help.
 Emeritus Sci Advisor PF Gold P: 16,091 Well, surely you can do something on it -- even if it's just rewriting the problem in a less opaque form. e.g. do you know anything about proving two subspaces equal? (Or two sets?)
P: 317
 Quote by loesch.19 Let U and V be subspaces of a vector space W. If W=U $$\oplus$$ V, show U $$\bigcap$$ V={0}. I'm a bit lost on this one... as I thought this was essentially the definition of direct sum. I'm unsure where to start. Any help would be great!
I think you should add the definition as follows:

If U and V are subspaces of a Vector space W and each $$w \in W$$ can be written as the unique sum as u+v where $$u \in U$$ and $$v \in V$$ then

W is the direct sum of U and V and can be written $$W = U \oplus V$$
 Math Emeritus Sci Advisor Thanks PF Gold P: 39,545 Suppose there were a non-zero vector, w, in both U and V and let u be any vector in U. Now, write u as two different sums of vectors in U and V.

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