第九版的英文答案里有这个题：
a.        Since  , the function is concave.
b.        Because  , and  , Equation 2.98 is satisfied and the function is concave.
c.        y is quasi-concave as is  .  But   is not concave for γ > 1.  All of these results can be shown by applying the various definitions to the partial derivatives of y.