[Python] Plotting defined function

I've defined a function which I would now like to plot:

import numpy as np
from math import pi, sqrt
import matplotlib.pyplot as plt

def f(x: float) -> float:
return pi * x * sqrt(x**2 + 400) + pi * x**2 - 1200

plt.plot(f(x))
plt.show()


When running this code, I get "NameError: name 'x' is not defined".

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