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

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