Given that the function z = Z (x, y) is determined by the equation x ^ 2 + y ^ 2 + Z ^ 2 = 2RZ, find DZ Please give the solution process, thank you

Given that the function z = Z (x, y) is determined by the equation x ^ 2 + y ^ 2 + Z ^ 2 = 2RZ, find DZ Please give the solution process, thank you

The partial derivative of X: 2x + 2zz'x = 2rz'x, z'x = x / (R-Z)
If the partial derivative of Y is 2Y + 2zz'y = 2rz'y, then z'y = Y / (R-Z)
So DZ = z'xdx + z'ydy = (xdx + YDY) / (R-Z)