Energy of an damped/undriven oscillator in terms of time?

[GreenTikiFire]

Homework Statement

The Q asks to show that the time rate of change in mechanical energy for a damped, undriven oscillator is dE/dt=-bV^2.

Homework Equations

I assume you take the derivative of the total E eq, E=(1/2)mV^2 + (1/2)kx^2 but I'm unsure how to put the E eq into terms of t, like E(t).

The Attempt at a Solution

Would you have to punch in the pos eq [x(t)=(Ae^(-βt))cos(ωt-δ)] in for x, then its derivative in for V? and then takes that entire eq's derivative? Seems like it would be too much work and not enough concept.

 

[GreenTikiFire]

i can't seem to get it in the form dE/dt=-bV^2.