10,11

Consider a rectangle $A B C D$ and a point $E$ not lying in its plane. Let the planes $A B E$ and $C D E$ intersect along the line $l$, and the planes $B C E$ and $A D E$ - along the line $p$. Find the angle between the lines $l$ and $p$.
Planes $A B E$ and $C D E$ pass through parallel lines $A B$ and $C D$ and intersect along line $l$, so $l \|$ $A B$. Similarly, line $p$ of the intersection of planes $B C E$ and $A D E$ is parallel to line $B C$. Since lines $A B$ and

BC are perpendicular, the lines $l$ and $p$ parallel to them are also perpendicular.

## Answer

$90^{\circ}$.