The total energy stored in the system is equal to the area OABO, which is in turn equal to OACO, therefore: E n e r g y = Area ⁡ ⁡ ( O A B O ) = W stored = ∫ ∫ 0 λ λ i ( λ λ ) d λ λ C o e n e r g y = Area ⁡ ⁡ ( O A C O ) = W stored ′ = ∫ ∫ 0 i λ λ ( i ) d i