SOLVED:Differentiate Equation 28.9 to find the current in the L C circuit, and use q=C V to find the voltage. From these, obtain the electric energy in the capacitor and the magnetic energy in the inductor, and sum to show that the total energy remains constant. (Hint: You'll need Equation 28.10 and a familiar trig identity.)

Differentiation

Differential Equations (Definition, Types, Order, Degree, Examples)