Let f be the function defined as follows. y = f(x) = 3x + 7 (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 4 to 4.1. (Round your answer to three decimal places.) dy = (c) Find the actual change in y if x changes from 4 to 4.1 and compare your result with that obtained in part (b). (Round your answer to three decimal places.) Δy =