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# A car travels along straight road for the first half-time with a velocity $v_1$ and the second half-time with velocity $v_2$ The average velocity of cars is

$\begin{array}{1 1}(A)\;\large\frac{v_1+v_2}{2} \\(B)\;\large\frac{2v_1v_2}{v_1+v_2}\\(C)\;\sqrt{v_1v_2}\\(D)\;\sqrt {\large\frac{v_2}{v_1}} \end{array}$

$v= \large\frac{total\;distance}{total\;time}$
$\quad= \large\frac{s}{\Large\frac{s/2}{v_1} +\frac{s/2}{v_2}}$
$\quad= \large\frac{2v_1v_2}{v_1+v_2}$
Hence B is the correct answer.