That's a problem of partial derivatives. Δ z/Δ x means that z takes the derivative of x and y takes it as a constant.

1.δz/δx=x^2 e^(x+y)+2x e^(x+y)

2.δz/δy=x^2 e^(x+y)

3.dz=(δz/δx)dx+(δz/δy)dy=x^2 e^(x+y)+2x e^(x+y)dx+x^2 e^(x+y)dy

Refer to/view/view/1029405.htm for details.