import numpy as np
import matplotlib.pyplot as plt


def f(x):
    return (x**2)/3

def dfdx(x):
    return 2*x/3

def tangent(x, punkt):
    return dfdx(punkt) + (x - punkt) + f(punkt)
    
a = 1.0
xa = np.linspace(0, 4, 100)

plt.close('all')
plt.plot(a, f(a),'ro', label='f(a)')
plt.plot(xa, f(xa), label='f(xa)')
plt.plot(xa, tangent(xa, a), label='tangent')
plt.legend()
plt.xlabel('x')
plt.grid()

# plt.savefig('plot_elev1.pdf')
plt.show()