code: def der(self, x):
      R = sqrt(x[0] ** 2 + x[1] ** 2)
      term1 = -20.0 * exp(-0.2 * R)
      term2 = -exp(0.5 * (cos(2.0 * pi * x[0]) + cos(2.0 * pi * x[1])))
  
      deriv1 = term1 * (-0.2 * 0.5 / R)
  
      dfdx = 2.0 * deriv1 * x[0] - term2 * pi * sin(2.0 * pi * x[0])
      dfdy = 2.0 * deriv1 * x[1] - term2 * pi * sin(2.0 * pi * x[1])
  
      return np.array([dfdx, dfdy])
  
imports: import time
  import numpy
  from numpy import sin, cos, pi, exp, sqrt, abs
  from scipy.optimize import rosen
label_desc: Use image node_id 2 for calling the Ackley obj's underlying member method code with example usage: obj.der(x) and returns: np