k is the force constant of a spring what will be the work done in increasing its extension form $l_1 to l_2$ be ?
At extension $l_1$ the stored energy = $ {1 \over 2 } kl_1^2 $ $ \therefore$ At extension $ l_2 $ the stored energy = ${1 \over 2} kl_1^2 $ Work done in increasing its extension from $l_1 to l_2 $ =$ {1 \over 2} k ( l_2 ^2 - l_1 ^2 ) $