If u = x / (x + y), find du/dx, du/dy. If st", find as/at, s/du. If z = ln /u2 + v + w, find dz/du, dz/dv, dz/dw. For w = xy-2xy +6, find 0w/dx and aw/dy at the points where dw/dx = dw/dy = 0. For w = 8x + y - 2xy, find aw/ax and 2 w/y at the points where dw/dx=dw/dy = 0. For u = e cos y, (a) verify that 8u/ry = d u/yx; (b) verify that u/dx +du/dy = 0.