Consider the function f(z) = 12z' + 15x' + 240z" + 4.
f(z) has inflection points at (reading from left to right) x = D, E, and F,
where D is less than E, and E is less than F.
For each of the following intervals, tell whether f(z) is concave up or concave down:
D): [select an answer (D,E)] Select an answer (E,F): Select an answer (F, 0)= Select an answer