因式分解(1
(1-X^2)(1-Y^2)-4XY=1 - X^2 - Y^2 + X^2 * Y^2 - 4XY=( 1 - 2XY + X^2 * Y^2 ) - ( X^2 + 2XY + Y^2 )=( 1 - XY )^2 - ( X + Y )^2=( 1 - XY + X + Y )( 1 - XY - X - Y )