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A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is $$f_1$$. If the speed of the train is reduced to 17 m/s, the frequency registered is $$f_2$$. If speed of sound is 340 m/s, then the ratio $$f_1/f_2$$ is:
$$f' = f_0 \left(\frac{v}{v - v_s}\right)$$
$$f_1 = f_0 \left(\frac{340}{340 - 34}\right) = f_0 \left(\frac{340}{306}\right)$$
$$f_2 = f_0 \left(\frac{340}{340 - 17}\right) = f_0 \left(\frac{340}{323}\right)$$
$$\frac{f_1}{f_2} = \frac{f_0 \left(\frac{340}{306}\right)}{f_0 \left(\frac{340}{323}\right)} = \frac{323}{306} = \frac{19 \times 17}{18 \times 17} = \frac{19}{18}$$
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