a1 a2 a3b1 b2 b3c1 c2 c3结果为 a1·b2·c3+b1·c2·a3+c1·a2·b3-a3·b2·c1-b3·c2·a1-c3·a2·b1(注意对角线就容易记住了）或可这么记 a1(b2·c3-b3·c2)+a2(b1·c3-b3·c1)+a3(b1·c2-b2·c1)
