KURSCHAK 1966

Can we arrange $5$ points in space to form a pentagon with equal sides such that the angle between each pair of adjacent edges is $90^o$ ?

Show that the $n$ digits after the decimal point in $(5 +\sqrt{26})^n$ are all equal.

Do there exist two infinite sets of non-negative integers such that every non-negative integer can be uniquely represented in the form $a + b$ with $a$ in $A$ and $b$ in $B$ ?