Let x,y,z,u,v,w∈R ,prove that \n\n 1.(x−u)2+(y−v)2+(z−w)2≥ \n 21((x−z)(x−u−v+z)+(y−x)(y−v−w+x)+(z−y)(z−w−u+y)) \n\n 2.(x−z)(x−w)+(y−x)(y−u)+(z−y)(z−v)≥2((x−u)(u−z)+(y−v)(v−x)+(w−y)(z−w)) \n\n 3.(x−z)(2x−u−z)+(y−x)(2y−v−x)+(z−y)(2z−w−y)≥2((v−w)(u−v)+(w−u)(v−w)+(w−u)(u−v))