z = f (x, y)
z`_(x)(x;y)=(4x^5-3x^2y^3+6y^5)`_(x)=4*(x^5)`_(x)-3y^3*(x^2)`_(x)+6(y^5)`_(x)=20x^4-6y^3x+0=20x^4-6xy^3
z`_(y)(x;y)=(4x^5-3x^2y^3+6y^5)`_(y)=4*(x^5)`_(y)-3x^2*(y^3)`_(y)+6(y^5)`_(y)=4*0-3x^2*3y^2+30y^4=30y^4-9x^2y^2
[b]dz=(20x^4-6xy^3)*dx+(30y^4-9x^2y^2)*dy[/b]