A friend hands you an equation sheet with the following equation for the Doppler effect:

. This version of the equation is correct with signs as given only if
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the observer and source are approaching each other.
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the observer is approaching the source while the source is moving away from the observer.
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the observer is moving away from the source while the source is approaching the observer.
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the observer and source are moving away from each other.
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the observer and source are moving in perpendicular directions.