δz/δv =δz/δx *δx/δv +δz/δy *δy/δv=(2xy-y^2)(-usinv)+(x^2-2xy)ucosv δz/δu =δz/δx *δx/δu +δz/δy *δy/δu=(2xy-y^2)cosv+(x^2-2xy)sinv
