4(ab+cd)²-(a²+b²-c²-d²)²
=2²(ab+cd)²-(a²+b²-c²-d²)²
=＜2(ab+cd)+(a²+b²-c²-d²)＞＜2(ab+cd)-(a²+b²-c²-d²)＞
=
=（a+b+c-d)（a+b-c+d)(c+d+a-b)（c+d-a+b）
16+24（a-b）+9(a-b)²
=4²+2*4*3(a-b)+3²(a-b)²
=²