If a spring extends by x on loading ,then energy stored by the spring is(if T is the tension in the spring and k is the spring constant)

work = Force.displacement dw =F.dx---(1)              in a spring F=-kx⇒ dF=-kdx⇒ -dFk=dx---(2) substitute eqn(2) in (1) dw =F(-dFk)⇒ integrating ∫dw =∫F(-dFk)⇒ w=-F22k here work w is energy stored F is tension in the spring T negative is for restoring force of spring ∴energy stored =T22k