Take a square with the size of 6×8. This 6×8 square must be covered by 16 3× 1 squares, and this square must exist. At this time, there is a 2×8 figure with a square missing. Since 2 < 3, there must be a 2×6 square that can be covered by 4 1×3 squares. And there is only one L-shaped square left, which must not be covered with a 3× 1