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Find the second partial derivatives of the following functions
a) z = 3x^2 − 4xy + 15y^2
b) z = 4xe^y
c) z = 6xln(y)


Sagot :

(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/∂xy = -4

∂²z/∂yx = -4

∂²z/∂y ² = 30

where ∂²z/∂xy = ∂/∂x [∂z/∂y] and ∂²z/∂yx = ∂/∂y [∂z/∂x].

(b) z = 4x

z/∂x = 4

z/∂y = 4x

∂²z/∂x ² = 0

∂²z/∂xy = 4

∂²z/∂yx = 4

∂²z/∂y ² = 4x

(c) z = 6x ln(y)

z/∂x = 6 ln(y)

z/∂y = 6x/y

∂²z/∂x ² = 0

∂²z/∂xy = 6/y

∂²z/∂yx = 6/y

∂²z/∂y ² = -6x/y ²