f = 2(x1-x2)^2 -x2^2-4x3^2-2x2x3= 2(x1-x2)^2 -(x2+x3)^2-3x3^2= 2y1^2 - y2^2 -3y3^2(A) 正确另: 适当加点悬赏, 答的人才会多!
