What is the "general" alpha value blending equation used in

• alpha blending;
• additive blending;
• multiplicative blending;
• 2x multiplicative blending?

Should this be 

(source.alpha × (1 - destination.alpha)) + (destination.alpha × 1)?

Case: source.alpha = 1 -> 1

Case: source.alpha = 0 -> destination.alpha

Case: source.alpha = destination.alpha = 0.5 -> 0.5 * 0.5 + 0.5 = 0.75