If L= X^a Y^b + lamda (p1(m - X) + p2(n - Y))

and:

dL/dx1= aX^a-1 Y^b - lamda p1

dL/dy1= bX^a Y^b-1 - Lamda p2

The cross partial derivative d^2L/ Dx1Dy1 = ab(x^a-1 y^b-2)

Where does the y^b-2 come from is the cross derivative? why is it not just y^b-1

Any explanation would be a big help. Cheers