c
\({\upsilon _{av}} = \frac{{{d_1} + {d_2}}}{{\frac{{{d_1}}}{{{\upsilon _1}}} + \frac{{{d_2}}}{{{\upsilon _2}}}}}\)= \(\frac{{\frac{d}{2} + \frac{d}{2}}}{{\frac{{d/2}}{{{\upsilon _1}}} + \frac{{d/2}}{{{\upsilon _2}}}}}\)
\(\Rightarrow {\upsilon _{av}} = \frac{2}{{\frac{1}{{{\upsilon _1}}} + \frac{1}{{{\upsilon _2}}}}}\) = \(\frac{{2{\upsilon _1}\,{\upsilon _2}}}{{{\upsilon _1} + {\upsilon _2}}}\)