αz/αx＝ln(xy)＋x×1/(xy)×y＝1＋ln(xy)
αz/αy＝x×1/(xy)×x＝x/y
α^2z/αx^2＝1/(xy)×y＝1/x
α^2z/αxαy＝1/(xy)×x＝1/y
α^2z/αy^2＝－x/y^2