1=xy/x+y,2=yz/y+z,3=zx/z+x,则x+y+z的值为（ ）.A.276/35 B.-276/35 C.11/12 D.-11/12

(x+y)/xy=1 (y+z)/yz=1/2 (z+x)/zx=1/3 就是说 1/x+1/y=1 1/y+1/z=1/2 1/z+1/x=1/3 上面分别代入下面1/x+1/y+1/z=11/12得出1/x=5/12,1/y=7/12,1/z=-1/12得出z=-12 y=12/7 x=12/5x+y+z=-276/35选B