If a body having mass M is suspended from the free ends of two springs A and B, their periodic time are found to be $T_1$ and $T_2$ respectively. If both these springs are now connected in series and if the same mass is suspended from the free end, then the periodic time is found to be T. Therefore …………..
$ T_1 = 2 \pi \sqrt { m^1 \over k_1 } \Rightarrow k_1 = { 4 \pi^2 M \over T_1^2 } and k_2 = { 4 \pi ^2 M \over T_2^2 } $ For series connection ; $ T = 2 \pi \sqrt { M \over k} where k = { k_1 k_2 \over k_1 + k_2 } $ $ \therefore T = 2 \pi \sqrt { {M^1 \over 4 \pi^2 M} ( T_1^2 + T_2 ^2 ) } $ $ \therefore T = \sqrt { T_1^2 + T_2 ^2 } \Rightarrow T^2 = T_1 ^2 + T_2 ^2 $