(a) $T \propto \frac{1}{\sqrt{k}} \Rightarrow T_{1}:T_{2}:T_{3} = \frac{1}{\sqrt{k}}:\frac{1}{\sqrt{k/2}}:\frac{1}{\sqrt{2k}} = 1:\sqrt{2}:\frac{1}{\sqrt{2}}$
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Ph.D. in Physics with 19+ years of teaching experience.