5. (a) Find the Taylor series of the function cosh I about r = 0 as far as the term in r', and use this series to estimate the value of the function at x = 0.2. (b) Given the first order increment formula where z = z(x, y), find the directional derivative of z in a direction making angle 0 with the r axis, at any point (z, y). For the function z(z, y) = x2- 2y'+ 3xy, find the rate of change of z at the point (1, 1) in a direction making angle 0 with the I-axis. Show that the rate of change is greatest when tan 0 = 2/5