两边同时lg
得到 lg[根号下(r1+r3)+根号下(r2+r3)]=lg[根号下(r1+r2)]
两边乘2得到 lg[根号下(r1+r3)+根号下(r2+r3)]^2=lg(r1+r2)
化简 lg{r1+r3+r2+r3+2*[根号下(r1+r3)*根号下(r2+r3)]}=lg(r1+r2)
去掉lg得到 {r1+r3+r2+r3+2*[根号下(r1+r3)*根号下(r2+r3)]}=r1+r2
化简 2*r3+2*[根号下(r1+r3)*根号下(r2+r3)]=0
则 r3+[根号下(r1+r3)*根号下(r2+r3)]=0
化简 r1*r2+r2*r3+r1*r3=0
r3=-r1*r2/(r1+r2)