Solve the p.d.e. 2r + 5s + 2t = 0 ⇒2∂2z∂x2 + 5∂2z∂x∂y + 2∂2z∂y2 = 0 ⇒2D2 + 5DD' + 2D'2 z = 0 Auxiliary equation 2m2 + 5m + 2 = 0 ⇒2m2 + 4m + m + 2 = 0 ⇒2mm + 2 + 1m + 2 = 0 ⇒m + 22m + 1 = 0 ⇒m =-2, -12 Solution: z = f1y - 2x + f2y - x2