title: Python curve fitting by specific polynominal
body: <p>I have a question about the Python curve fitting,
  I know there's polyfit function in numpy, but if I assign the polynomial as
  A<em>X^4 + B</em>X^2, how to find this A and B???</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  points = np.array([(1, 1), (2, 4), (3, 1), (9, 3)])
  # get x and y vectors
  x = points[:,0]
  y = points[:,1]
  
  # calculate polynomial
  z = np.polyfit(x, y, 4)  &lt;---?
  f = np.poly1d(z)         &lt;---?   
  </code></pre>
  
  <p>Could someone give a hint???
  Thank you!</p>