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Let V be a finite dimensional vector space over K and W be a subspace of V. Then which of the following statements is true ?

  • A.\(\dim V = \dim W + \dim \dfrac{W}{V}\)
  • B.\(\dim V = \dim W - \dim \dfrac{V}{W}\)
  • C.\(\dim V = \dim W + \dim \dfrac{V}{W}\)
  • D.\(\dim V = \dim \dfrac{V}{W}\)

Answer: C

The true statement is \(\dim V = \dim W + \dim V/W\).

The quotient map \(V \to V/W\) is onto with kernel W, so by rank-nullity \(\dim V = \dim W + \dim V/W\). Equivalently, extend a basis of W to a basis of V; the added vectors give a basis of V/W.

W/V is not defined, since W is the smaller space. A minus sign, or leaving out \(\dim W\), gives the wrong count; for \(V = \mathbf{R}^{3}\) and W a plane, \(\dim V/W = 1\) and only \(3 = 2 + 1\) works.

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