Please explain specific and clear Thank you:)

4. We are going to define one final function, D (x) = So 0.2t sin (cos (sint)) dt. f (1) + Don't worry about trying to find a simple formula for D (x). But, using our amazing fact, fill in the last blank: D' (x) = Not only is the fact that we've discovered a surprising one; as we shall see in Section 5.4, it is also extremely useful