ASK YOUR TEACH Let f be the function defined as follows. y = f(x) = 7x2- 4x+9 (a) Find the differential of f. dy = (b) Use your result from part (a) to find the approximate change in y if x changes from 2 to 1.97. (Round your answer to two decimal places.) dy = (c) Find the actual change in y if x changes from 2 to 1.97 and compare your result with that obtained in part (b). (Round your answer to two decimal places.) Ay =