It's possible to annotate certain points by applying the annotate function.
x = 3 * np.pi / 4
Plot vertical line:
plot([x,x],[0,3 * np.sin(x)], color ='blue', linewidth=2.5, linestyle="--")
Print the blue dot:
scatter([x,],[3 * np.sin(x),], 50, color ='blue')
annotate(r'$3\sin(\frac{3\pi}{4})=\frac{3}{\sqrt{2}}$',
xy=(x, 3 * np.sin(x)),
xycoords='data',
xytext=(+10, +30),
textcoords='offset points',
fontsize=16)