David:

Thanks for spotting that.

We could correct the problem as you suggest. But because Theorems A2.1 and A2.4 to which the exercise refers are phrased in terms of the function's concavity, let's just solve it this way:

In Exercise A2.20, page 511, replace "f(x)=x^4 ..." with "f(x)=-x^4 ..."

(note the insertion of the minus sign)


The phrasing of the question can now stand as it is.

Thanks again, and keep them coming..