In series, effective spring constant, \(k = \frac{{{k_1}{k_2}}}{{{k_1} + {k_2}}}\)
\(\therefore \) Time period, \(T = 2\,\pi \sqrt {\frac{{m\,({k_1} + {k_2})}}{{{k_1}{k_2}}}} \) …..(ii)
Now, \(t_1^2 + t_2^2 = 4\,{\pi ^2}m\,\left( {\frac{1}{{{k_1}}} + \frac{1}{{{k_2}}}} \right) = \frac{{4\,{\pi ^2}m\,({k_1} + {k_2})}}{{{k_1}{k_2}}}\)
\(t_1^2 + t_2^2 = {T^2}.\) [Using equation (ii)]