import numpy as np
import matplotlib.pyplot as plt

n = np.arange(0,200)

a = 0.9

# x[n] = a^n u[n]
x = a**n * np.heaviside(n, 1)

asums = np.zeros(len(n))

for i in range(len(n)):
    asums[i] = np.sum(x[0:i])

# Plot settings for $x[n]$
plt.subplot(121)
plt.plot(n, x)
plt.xlabel("$n$")
plt.ylabel("$x[n]$", rotation="horizontal")

# Plot settings for sum of $x[n]$
plt.subplot(122)
plt.plot(n, asums)
plt.xlabel("$n$")
plt.ylabel("$\sum_n x[n]$", rotation="horizontal")
plt.show()