12. If f'(x): = 0 then f(x) is neither increasing or decreasing, it's a critical point and the slope of the tangent line is a vertical line. 13. A cubic function has no absolute maximum or minimum in an open interval. 14. A quartic function has no absolute maximum or minimum in an open interval. 15. An inflection point of a function must occur where the second derivative of the function changes sign. 16. 17. If the first derivative of a function is positive over an interval, then the function must be increasing over that interval. If a function has a relative maximum at a certain point, then the second derivative of the function must be positive at that point.