定理 设实数x,y,z满足 yz+zx+xy=x+y+z+t,则有(x-k)^2+(y-k)^2+(z-k)^2≥2k^2-2k-1-2t (1)定理证明 (1)式展开为k^2-2k(x+y+z-1)+x^2+y^2+z^2+2t+1≥0k^2-2k(x+y+z-1)+(x+y+z)^2-2(x+y+z)+1≥0k^2-2k(x+y+z-1)+(x+y+z-...