U=12kx\mathrm{U}=\frac{1}{2} \mathrm{kx}U=21kx
U=kx\mathbf{U}=\mathbf{k x}U=kx
U=12kx2\mathrm{U}=\frac{1}{2} \mathrm{kx}^{2}U=21kx2
U=kx2\mathrm{U}=\mathrm{kx}^{2}U=kx2