... =+−+=+−+−=22)(2)2)(2(2)(2)2)(2(12)(212)(11.2)('xzyxzyyxzyxzyyxzyyxzyxzyzF−+−++−=−+−++−=−−+−=−−+−−=2222222222)('''1)())(()())(('''xzyyxzFFzxzyxzyyxzyxzyyFFzzyyzxx−++−=−==−+−++−−+−++−=−=→ nnª VËydyxzyyxzdxdyzdxzdyxyxz22),()(''−++−+=+= dã, DoC u 11 : (1đ) cho hàm ẩn),( zyxx=có PT :23xyxx4z+−= Víi243),,(xyxxzzyxF−−+=⇔ 1'2'43'22=−=−−=zFxyFyxFdyx...