# dout: ∂L/∂o
  # a:    the input of FC layer
  # the shape of a: (batch size, hidden layer neuron)
  # the shape of dw: (hidden layer neuron, num of classes)
  dw = a.reshape(a.shape[0], -1).T.dot(dout)
  db = np.sum(dout, axis=0)