Evaluate f(1, 2) and f(1.05, 2.1) and calculate Δz. Also, use the total differential dz to approximate Δz.

f(x, y) = x siny

z_x = siny

z_x = xcosy

dz = sinydx + xcosydy

To find f(1, 2) and f(1.05, 2.1), I evaluate the given function at the two points.

I am having trouble finding delta z and then using dz to approximate delta z.

The answer for delta z is given to be -0.00923 and the numerical answer for dz is given to be 0.00385.

How is this done?