15. Find the curvature and torsion of curves with the following unit tangent vectors, given that s denotes arc length. (Hint: d sech s = sech s+tanhs= = 1.) (a) u = (tanhs, sech s, 1) tanhs= - sech s tanhs, sechs, and (b) u= (- sin s tanhs, cos s tanhs, sech s).