x=2,y=1,z=2/√5
x=2.01,y=1.03,z=2.01*1.03/√(2.01^2+1.03^2)=0.916654385...
,△Z=0.916654385-2/√5=,0.02222
z'x=y^3/(x^2+y^2)^(3/2)x=2,y=1,z'x=1/5^(3/2)
z'y=x^3/(x^2+y^2)^(3/2),x=2,y=1,z'y=8/5^(3/2)
dz=(dx+8dy)/5^(3/2)=(0.01+0.24)/5^(3/2)=(√5)/100=0.02236