Chapter 15 - Differentiation in Several Variables - 15.3 Partial Derivatives - Preliminary Questions - Page 780: 3

(a) and (c)

(a) should be evaluated without using the Quotient Rule because we are deriving with respect to $x$ and there is no $x$ in the denominator. (b) should be evaluated using the Quotient Rule because we are deriving with respect to $y$ and there is a $y$ in the denominator. (c) should be evaluated without using the Quotient Rule because we are deriving with respect to $x$ and there is no $x$ in the denominator.