a-b=3/5b-c=3/5相加a-c=6/5所以(a-b)^2+(b-c)^2+(c-a)^2=(3/5)^2+(3/5)^2+(6/5)^2=54/25所以a^2-2ab+b^2+b^2-2bc+c^2+a^2-2ac+c^2=54/252(a^2+b^2+c^2)-2(ba+ca+bc)=54/252*1-2(ba+ca+bc)=54/25ba+ca+bc=(2-54/25)/2...