a+b+c=0a+b=-c(a+b)^2=c^2a^2+b^2-c^2=-2ab同理：b^2+c^2-a^2=-2bc,c^2+a^2-b^2=-2ac1/(b^2+c^2-a^2)+1/(c^2+a^2-b^2)+1/(a^2+b^2-c^2)=-1/(2bc)-1/(2ac)-1/(2ab)=-(a+b+c)/(2abc)=0
