## Task 4 - 110914

In a plane $\varepsilon$ lies a rectangle $A B C D . S$ is a point on the perpendicular in $A$ to $\varepsilon$.

Determine the size of the angle $\angle C D S$!
The points $A, D, S$ lie in a plane $\varepsilon^{\prime}$, which is perpendicular to the plane $\varepsilon$ through $A, B, C$ and $D$; for by assumption, $A D \perp A B$ and, if $S \neq A$, also $S A \perp A B$.

In the case $S=A$, let $\varepsilon^{\prime}$ be the plane perpendicular to $\varepsilon$ through $A$ and $D$. Therefore, in any case, $C D \perp \varepsilon^{\prime}$ and thus $|\angle C D S|=90^{\circ}$.