（c²-b²+d²-a²）²-4（ab-cd）²
=[（c²-b²+d²-a²）+2（ab-cd）][（c²-b²+d²-a²）-2（ab-cd）]
=[(c²-2cd+d²)-(a²-2ab+b²)][(c²+2cd+d²)-(a²+2ab+b²)]
=[(c-d)²-(a-b)²][(c+d)²-(a+b)²]
=(c-d+a-b)(c-d-a+b)(c+d+a+b)(c+d-a-b)