Commands for important formulas for linear algebra

[wunderlich]

Commands for important formulas for linear algebra should be added. E. g.:

Planes in normal form (not using xparse and l3):

\newcommand{\planen}[3][]{%
  {\ensuremath%
    \ifthenelse{\equal{#1}{\empty}}{}{
      #1:
    }
  }
  \left[ \vec{x} - \vect{#2} \right] \cdot \vect{#3} = 0
}

Planes and lines in parametrical form:

\newcommand{\planep}[4][]{
  {\ensuremath%
    \ifthenelse{\equal{#1}{\empty}}{}{
      #1:
    }
  }
  \vec{x} = \vect{#2} + r \vect{#3} + s \vect{#4}
}

\newcommand{\linep}[3][]{
  {\ensuremath%
    \ifthenelse{\equal{#1}{\empty}}{}{
      #1:
    }
  }
  \vec{x} = \vect{#2} + r \vect{#3}
}