Question

u= ln(x^4+y^4/x+y)
then prove that
xdu/dx +ydu/dy =3

Answer 1

u=ln(x^4+y^4/x+y)
u=ln(x^4+y^4)-ln(x+y)
so
xdu/dx=4x^4/(x^4+y^4) -1
ydu/dy=4y^4/(x^4+y^4) -1
xdu/dx+ydu/dy=in photo
it's equals to 2 not 3