CJMO 2022

Let $\triangle{ABC}$ has circumcircle $\Gamma$ , drop the perpendicular line from $A$ to $BC$ and meet $\Gamma$ at point $D$ , similarly, altitude from $B$ to $AC$ meets $\Gamma$ at $E$ . Prove that if $AB=DE, \angle{ACB}=60^{\circ}$ (sorry it is from my memory I can't remember the exact problem, but it means the same)

You have an infinite stack of T-shaped tetrominoes (composed of four squares of side length 1), and an n × n board. You are allowed to place some tetrominoes on the board, possibly rotated, as long as no two tetrominoes overlap and no tetrominoes extend off the board. For which values of n can you cover the entire board?