4a+b+c+d/a =a+4b+c+d/b =a+b+4c+d/c =a+b+c+4d/d
Suy ra: 3a+a+b+c+d/a =3b+a+b+c+d/b =3c+a+b+c+d/c =3d+a+b+c+d/d
Suy ra: 3+a+b+c+d/a =3+a+b+c+d/b =3+a+b+c+d/c =3+a+b+c+d
Suy ra: a+b+c+d/a =a+b+c+d/b =a+b+c+d/c =a+b+c+d/d (1)
TH1:a+b+c+d=0 suy ra P=-4
TH2: a+b+c+d khác 0 suy ra P=4