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What is the span of three vectors in 3D space when the third vector pokes out of the plane?

When a third vector pokes out of the plane at an angle, scaling it allows the entire 2D sheet to slide up and down through space. With three freely varying scalars (av⃗+bw⃗+cu⃗a\vec{v} + b\vec{w} + c\vec{u}), the span now encompasses every conceivable point in the 3D volume.

Conditions

  • The first two vectors span a plane in 3D space.
  • The third vector is not on that plane (it pokes out at an angle).
  • The scalars aa, bb, and cc vary freely.

Reasoning, step by step

  1. Start with two vectors that span a 2D plane in 3D space.
  2. Introduce a third vector that points outside this plane.
  3. Scale the third vector to slide the 2D plane up and down.
  4. Allow all three scalars to vary freely in the combination av⃗+bw⃗+cu⃗a\vec{v} + b\vec{w} + c\vec{u}.
  5. Conclude that the span encompasses the entire 3D volume.

Example

With three freely varying scalars (av⃗+bw⃗+cu⃗a\vec{v} + b\vec{w} + c\vec{u}), the span encompasses every conceivable point in 3D volume.

Common misconceptions

  • Believing the span remains a 2D plane.
  • Thinking the third vector must be orthogonal to the plane to expand the span.

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