xyz +√(x^2+y^2+z^2)=√2yz.dx+xz.dy +xy.dz +[ (xdx+ydy+zdz)/√(x^2+y^2+z^2) ]=0(x,y,z)=(1,0,-1)0 -dy +0 + [ (dx+0-dz)/√(1^2+0+(-1)^2) ]=0-dy + (dx-dz)/√2 =0-√2dy + (dx-dz) =0dz = dx -√2dyans : A
