Problem C9:
A car is traveling with a velocity of v/n_{2}**i**, and the driver honks the
horn. The frequency of the horn is 640 Hz. You are driving your car with
a velocity v/n_{1}**i**, where in both cases v is the speed
of sound in air. The unit vector **i** points to the right. See the figure below.
What is the frequency of the horn's sound that you hear? If you hear a frequency of n_{3} Hz,
what is n_{3}? Note that n_{1} and n_{2} are unitless,
and n_{3} has units of Hz.

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