3. (a) Evaluate when : = dx and y = r - 31. (b) For the example in (a) find two values of r such that dy = 0. dx (c) Use central difference approximation dy y(x + h) - y(x - h) 2h to estimate the first derivative of y = x3 - 3r at r = 091 91 by taking h = 12 (d) What is the true percentage relative error