(a) z = 3x ² - 4xy + 15y ²
has first-order partial derivatives
∂z/∂x = 6x - 4y
∂z/∂y = -4x + 30y
and thus second-order partial derivatives
∂²z/∂x ² = 6
∂²z/∂x∂y = -4
∂²z/∂y∂x = -4
∂²z/∂y ² = 30
where ∂²z/∂x∂y = ∂/∂x [∂z/∂y] and ∂²z/∂y∂x = ∂/∂y [∂z/∂x].
(b) z = 4x eʸ
∂z/∂x = 4eʸ
∂z/∂y = 4x eʸ
∂²z/∂x ² = 0
∂²z/∂x∂y = 4eʸ
∂²z/∂y∂x = 4eʸ
∂²z/∂y ² = 4x eʸ
(c) z = 6x ln(y)
∂z/∂x = 6 ln(y)
∂z/∂y = 6x/y
∂²z/∂x ² = 0
∂²z/∂x∂y = 6/y
∂²z/∂y∂x = 6/y
∂²z/∂y ² = -6x/y ²