# 3.In a resonance air column experiment,first and second resonances are obtained at lengths of air columns L1 and L2, the third resonance will be obtained at a length of ---------

$\frac{\lambda }{2}={L}_{2}-{L}_{1}\phantom{\rule{0ex}{0ex}}⇒\lambda =2\left({L}_{2}-{L}_{1}\right)$
The third resonance will be obtained at a length of
${L}_{3}=\frac{5}{4}\lambda =\frac{5}{4}×2×\left({L}_{2}-{L}_{1}\right)=\frac{5}{2}\left({L}_{2}-{L}_{1}\right)$

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