The work done by external agent in stretching a spring of force constant k = 100 N/m from deformation x1 = 10 to deformation x2 = 20 cm.
U2 = 12kx22
U1 = 12kx21
Wext = ∆U = U2-U1
= 12kx22-12kx12 = 12k(x2 2-x12)
= 12×100×((20100)2-(10100)2) =150 J