[tex]\it a)\ \ \left.\begin{aligned}\it a\perp c\\ \\ b\perp c\end{aligned}\right\} \Rightarrow a||b\\ \\ \\ b)\\ \\ a\cap c\ne \varnothing \\ \\ b\cap c\ne \varnothing \\ \\ a\cap d\ne \varnothing \\ \\ b\cap d\ne \varnothing \\ \\ c\cap d = \{O\} \\ \\ c)\\ \\ a\perp c\\ \\ b\perp c[/tex]
