# 编辑代码

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0,10,1000)
y = np.cos(2 * np.pi * x) * np.exp(-x) + 0.8
plt.plot(x,y,'k',color = 'r',label = "$exp-decay$",linewidth =3)
plt.axis([0,6,0,1.8])
ix = (x>1.8) & (x<3)
plt.fill_between(x,y,0,where=ix,facecolor='grey',alpha=0.25)
plt.text(0.5 * (1.8 + 3),0.2,r"$\int_a^b f(x)\mathrm{d}x$",horizontalalignment = 'center')

plt.legend()
plt.show()