Use the small increments formula to estimate the change in z = x²y⁴ − x⁶ + 4y when
(a) x increases from 1 to 1.1 and y remains fixed at 0; 

(b) x remains fixed at 1 and y decreases from 0 to −0.5;
(c) x increases from 1 to 1.1 and y decreases from 0 to −0.5.