Question 16****+Find the acceleration for \(s(t)=A\cos(\omega t)\), where \(A\) and \(\omega\) are constants.
Question 18*****A student says \(f''(x)\) is always positive because it is a derivative of a derivative. Give a counterexample.
Question 19*****Find all \(x\) where \(f(x)=x^4-4x^3\) has zero second derivative, and interpret them as possible inflection points.