The source producing sound and an observer both are moving along the direction of propagation of sound waves. If the respective velocities of sound, source and an observer are $v, {v_s}$ and ${v_o}$, then the apparent frequency heard by the observer will be ($n =$ frequency of sound)
A$\frac{{n(v + {v_o})}}{{v - {v_o}}}$
B$\frac{{n(v - {v_o})}}{{v - {v_s}}}$
C$\frac{{n(v - {v_o})}}{{v + {v_s}}}$
D$\frac{{n(v + {v_o})}}{{v + {v_s}}}$
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B$\frac{{n(v - {v_o})}}{{v - {v_s}}}$
b (b) As both observer and source are moving in the direction of sound, so the velocities are as per sing convention are positive.
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