u=sin(y+ax)−(y+ax)2 ......(i) On differentiating partially w.r.t. x, we get ux=cos(y+ax)‌a−2(y+ax)a Again differentiating partially w.r.t. x, we get uxx=−sin(y+ax)‌a2−2a2 ........(ii) On differentiating partially Eq. (i) w.r.t. y,we get uy=cos(y+ax)−2(y+ax) ⇒ uyy=−sin(y+ax)−2 .......(iii) ∴ From Eqs. (ii) and (iii), we get uxx=a2uyy