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Mathematics
OpenStudy (anonymous):

Let U and V be subspaces of a vector space W. Prove that their intersection is also a subspace of W

OpenStudy (anonymous):

Let X be the intersection of U and V. 1) X closed under addition: Assume x in X and y in X. From this, we know x in U and y in U. But since we know that U is a subspace, x+y in U holds. Similarly, one can show that x+y in V and therefore x+y in X. So X is indeed closed under addition. 2) X closed under scalar multiplication: ------------------------------------------ Assume x in X and r in R. Again, we know x in U. Since U is a subspace, it is closed under scalar multiplication. Therefore, r*x in U holds. Also r*x in V holds with a similar argument. From this r*x in X follows. So X is closed under scalar multiplication. Therefore by subspace theorem, we're done. You can use the definitely of a vectorspace as well, but that's more tedious.

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