xy/(x+y)=6/5① ---->(x+y)/(xy)=5/6 1/x+1/y =5/6 (4)yz/(y+z)=12/7 ② 1/y+1/z= 7/12 (5)xz/(x+z)=4/3 1/x +1/z=3/4 (6)（4）+（5）+（6）得1/x+1/y+1/z=13/12 (7)(7)-(5)得 x=2(7)-(6) 得 y=3(7)-(4)得 z=4...