A square table of size 1001x1001 is filled with the numbers 1; 2; 3; ... ; 1001 in such a way that in every row
and every column all those numbers appear. If the table is symmetric with respect to one of its diagonals,
prove that in this diagonal all of the numbers 1; 2; 3; ... ; 1001 appear.
Let ABCD be a parallelogram with acute angle at A. Let E be the foot of the perpendicular from C to the
line AB and let F be the foot of the perpendicular from C to the line AD.