(a²-b²-c²+2bc)/(b²-c²-a²-2ac)=[(a²-(b²+c²-2bc)]/[b²-(c²+a²+2ac)]= =[(a²-(b-c)²]/[b²-(a+c)²]=(a-b+c)(a+b-c)/(b-a-c)(b+a+c)=-(a+b-c)/(b+a+c)= =(c-a-b)/(b+a+c)