\[\begin{array}{l}
a)\,\,\,Ta\,\,co:\,\,\,\left\{ \begin{array}{l}
\angle xOt + \angle tOz = {90^0}\,\,\,\,\left( {do\,\,\,Ox \bot Oz} \right)\\
\angle yOz + \angle tOz = {90^0}\,\,\,\left( {do\,\,\,Ot \bot Oz} \right)
\end{array} \right.\\
\Rightarrow \angle xOt = \angle yOz\,\,\,\,\left( {cung\,\,\,phu\,\,\,voi\,\,\angle tOz} \right).\\
b)\,\,\,Ta\,\,co:\,\,\left\{ \begin{array}{l}
\angle xOt + \angle tOz = {90^0}\\
\angle yOz + \angle tOz = {90^0}
\end{array} \right.\\
\Rightarrow \angle xOt + \angle tOz + \angle yOz + \angle tOz = {180^0}\\
\Leftrightarrow \left( {\angle xOt + \angle tOz + \angle yOz} \right) + \angle tOz = {180^0}\\
\Leftrightarrow \angle xOy + \angle tOz = {180^0}.\,\,\,\,\,\,\,\left( {cmt} \right)
\end{array}\]
