U=ln (x^2+y^2)найти деференциал

[latex]dz(x,y)= \frac{\partial z(x,y)}{\partial x} dx+ \frac{\partial z(x,y)}{\partial y} dy=[/latex] [latex]=(ln(x^2+y^2))'_xdx+ (ln(x^2+y^2))'_ydy=[/latex] [latex]= \frac{(x^2+y^2)'_x}{x^2+y^2} dx+ \frac{(x^2+y^2)'_y}{x^2+y^2} dy=[/latex] [latex]= \frac{2x}{x^2+y^2} dx+ \frac{2y}{x^2+y^2} dy[/latex]