def f(n,x):
    i=0
    s=0
    while i<=n:
        i=i+1
        s=s+np.cos(3*x)/(2**i)
    return s

import numpy as np 
import matplotlib.pyplot as plt
x=np.linspace(0,2*np.pi,1000)
y=f(300,x)
plt.plot(x,y)
plt.show()