Calculate the value of the differential, dz, and the value of the increment deltaz, for the surface f(x,y)=3x²xy+y² from point (2,1) to point (1.8,1.1).
a) dz = 0.78, deltaz = 0.022
b) dz = 0.54, deltaz = 0.015
c) dz = 0.66, deltaz = 0.018
d) dz = 0.42, deltaz = 0.012