Skew lines
Lines that are neither parallel nor intersect, are called skew.
Requirements
- Direction vectors are not collinear
- A false statement results when we equate the linear equations
Example
$\text{g: } \vec{x} = \begin{pmatrix} 2 \\ 4 \\ 6 \end{pmatrix} + r \cdot \begin{pmatrix} 0 \\ 4 \\ 0 \end{pmatrix}$
$\text{h: } \vec{x} = \begin{pmatrix} 6 \\ 4 \\ 8 \end{pmatrix} + s \cdot \begin{pmatrix} 0 \\ 0 \\ 8 \end{pmatrix}$
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Direction vectors not collinear?
First, examine if the direction vectors are multiples of each other (=collinear).$\vec{a}=t\cdot\vec{b}$
$\begin{pmatrix} 0 \\ 4 \\ 0\end{pmatrix}=t\cdot\begin{pmatrix} 0 \\ 0 \\ 8 \end{pmatrix}$
Determine $t$ for every row
- $0=t\cdot0$
- $4=t\cdot0$
- $0=t\cdot8$
- $0=0$
- $4=0$
- $t=0$
A contradiction results in the second row. Thus the direction vectors are not collinear.
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Equate $g$ and $h$
By equating the linear equations we try to find a point of intersection. If a contradiction occurs the lines are skew.$\begin{pmatrix} 2 \\ 4 \\ 6 \end{pmatrix} + r \cdot \begin{pmatrix} 0 \\ 4 \\ 0 \end{pmatrix}$ $= \begin{pmatrix} 6 \\ 4 \\ 8 \end{pmatrix} + s \cdot \begin{pmatrix} 0 \\ 0 \\ 8 \end{pmatrix}$
Now we set up an equation system and solve it.
- $2+r\cdot0=6+s\cdot0$
- $4+r\cdot4=4+s\cdot0$
- $6+r\cdot0=8+s\cdot8$
- $2=6$
- $4+4r=4$
- $6=8+8s$
A contradiction results from I. (false statement)
=> The lines are skew