MathNet 2015-part-05 — 191 problems (EN–VI) Question 12

Consider a triangular pyramidal frustum $ABCA'B'C'$. Points $D \in (AA')$, $E \in (BB')$ and $F \in (CC')$ are such that the planes $(AEF)$ and $(DB'C')$ are parallel. Prove that the planes $(A'EF)$ and $(DBC)$ are also parallel.