Răspuns :
[tex]\it a)\\ \\ \\ \left.\begin{aligned}\it AB||CD\ (din\ dreptunghiul\ ABCD)\ \ \ \ \\ \\ BB'||DD'\ (din\ ipotez\breve a)\ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ \\ AB,\ CD -concurente;\ BB',\ DD' -concurente\end{aligned}\right\} \Rightarrow (ABB')||(CDD')[/tex]
b) AA' = BB' = CC' = DD' = 10cm ⇒ ABCDA'B'C'D' - paralelipiped ⇒
⇒ (ABC) || (A'B'D')