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Homework Help: Linear Algebra showing a subspace

  1. May 8, 2016 #1
    I'm having trouble getting started on this one and I'd really appreciate some hints. This question comes from Macdonald's Linear and Geometric Algebra book that I'm using for self study, problem 2.2.4.

    1. The problem statement, all variables and given/known data
    Let U1 and U2 be subspaces of a vector space V.
    Let U be the set of all vectors of the form u1 + u2, where u1 is a member of U1 and u2 is a member of U2.
    Show that U is a subspace of V.

    2. Relevant equations
    U inherits it's operations from V (through U1 and U2) and so we have to show that scalar.mult and vec.add are closed for U.
    av + bv = (a+b)v

    3. The attempt at a solution
    Since U1 and U2 are a subset of V, then adding any u1 to u2 may not be closed wrt either U1 or U2 but I think they should be closed wrt V. Intuitively that seems true but I get stuck trying to be more concrete than that.
  2. jcsd
  3. May 8, 2016 #2


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    It often helps to write down explicitly what you're trying to show. For example, one of the things you want to show is if ##\vec{x},\vec{y} \in U##, then ##\vec{x}+\vec{y}\in U##.

    When you say ##\vec{x} \in U##, it means there are vectors ##\vec{x}_1 \in U_1## and ##\vec{x}_2 \in U_2##, such that ##\vec{x} = \vec{x}_1 + \vec{x}_2##. And so on…
  4. May 8, 2016 #3


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    Can you say what it means for ##U## to be a subspace of ##V##?

    Hint: Let ##u, v \in U## and ##a## be a scalar ...

    And @vela has given you even more of a helping hand!
  5. May 8, 2016 #4


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    We define:

    a and b vectors, W1 and W2 subsets of a vectorspace V.

    Definition: W1 + W2 = {a+b|a ∈ W1 and b ∈ W2}

    Choose 2 vectors a,b element of W1. Choose 2 vectors c,d elements of W2. W1 and W2 are subsets of a vectorspace V.
    Then $$a + c ∈ (W1 + W2)$$ and $$b + d ∈ (W1 + W2)$$. Try to continue.

    You need to proof that (a+b) + (c+d) ∈ (W1 + W2), this means: a + b ∈ W1 and c + d ∈ W2
  6. May 8, 2016 #5


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    That's giving too much away. You need to let the OP do the problem himself.
  7. May 8, 2016 #6
    What an amazing community.
    Thanks everyone for the replies! I'll take it from here
  8. May 8, 2016 #7


    Staff: Mentor

    So noted, and addressed. I would have deleted the offending post, but the OP has already seen it.
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