Vector perpendicular to two directions shown to be parallel to a vector triple product

Vector perpendicular to two directions shown to be parallel to a vector triple product

IB Year 6 | Grade 12
8 marks

The vectors \(\mathbf{p}\) and \(\mathbf{q}\) in three dimensions are non-zero and are not parallel.

The vector \(\mathbf{u}\) is given by \(\mathbf{u}=|\mathbf{q}|^2\,\mathbf{p}-(\mathbf{p}\cdot\mathbf{q})\,\mathbf{q}\), and \(\mathbf{u}\ne\mathbf{0}\).

  1. Show that \(\mathbf{u}\) is perpendicular to \(\mathbf{q}\).[2]
  2. Let \(\mathbf{n}=\mathbf{p}\times\mathbf{q}\), which is a non-zero vector.
    1. Explain why \(\mathbf{p}\cdot\mathbf{n}=0\) and \(\mathbf{q}\cdot\mathbf{n}=0\).
    2. Show that \(\mathbf{u}\cdot\mathbf{n}=0\).[4]
  3. Hence, state how the vectors \(\mathbf{u}\) and \(\mathbf{q}\times\mathbf{n}\) are related geometrically, giving a brief reason.[2]

Solution:

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Answer:(c) \(\mathbf{u}\) is parallel to \(\mathbf{q}\times\mathbf{n}\)

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