x(a-b)分之c:c/[x(a-b)]=c·y(b-c) · z(c-a)/[x(a-b)]
y(b-c)分之a:a/[y(b-c)]=a·x(a-b) ·z(c-a)/[y(b-c)]
z(c-a)分之b:b/[z(c-a)]=a·x(a-b) ·cy(b-c)/[z(c-a)]