Tips och lösning till U 7.6i

DD^t=\left(\begin{array}{r}3\\2\\1\end{array}\right) _{3\times\underline{1}} \left(\begin{array}{rrr} 3& 2 & 1\end{array}\right) _{\underline{1}\times3}= \begin{pmatrix}

 \begin{matrix}
   3\cdot 3 = 9 &\vert& 3\cdot 2=6& \vert& 3\cdot 1=3\phantom{1}\\
 \end{matrix}\\[-3pt]
 \rule 180pt 0.4pt 0pt\\
 \begin{matrix}
   2\cdot 3 = 6 &\vert& 2\cdot 2=4& \vert& 2\cdot 1=2\phantom{1}\\
 \end{matrix}\\[-3pt]
 \rule 180pt 0.4pt 0pt\\
 \begin{matrix}
   1\cdot 3 = 3 &\vert& 1\cdot 2=2& \vert& 1\cdot 1=1\\
 \end{matrix}

\end{pmatrix}_{3\times3},

DD^t=\left(\begin{array}{rrr} 9&6&3\\ 6&4&2\\3& 2&1\end{array}\right).