In the limit of weak damping ( γ/ω << 1) and small angles, show that the total energy (sum of kinetic and potential energy) of a pendulum described by equations x(t) = x0e^-γt/2 cos (ωrt + α) and v(t) = -x0ωe^-γt/2 sin(ωrt + α) is constant over one period, but decays in time proportional to e-γt.