The revenue from the sale of x units of a sedative and y units of an antibiotic is given by R(x, y) = 5x² + 9y² - 4xy. Suppose 9 units of sedative and 5 units of antibiotic are sold.

(a) What is the approximate effect on revenue if 10 units of sedative and 5 units of antibiotic are sold?
(b) What is the approximate effect on revenue if the amount of antibiotic sold is increased to 6 units, while sedative sales remain constant?