THERE IS AN INTEGRAL NUMBER IN EACH CELL OF AN N × N TABLE. IN EACH...

2.

There is an integral number in each cell of an

n

×

n

table. In each move, we may

change the signs of all the numbers in any row or column. Prove that after a finite

number of such moves, it is possible to have the sum of the numbers in each row

and column to be non-negative.

median BM. Prove that

∠

ABC

≤ °

60

.

A

M

B

H

C