I'm having a difficult time understanding the formulas for maximum constructive interference and perfect destructive interference. I know that MCI there's the biggest amplitude? and for PDI it's 0.

Question

The forumlas are

delta phi = 2pi(deltax/lambda)+delta phi
FOR BOTH OF THEM?? I don't get that.

Answer 1

Concentrate on how construcitve and destructive occur. Both are the same, waves same frequency, in phase, or exactly 180 degrees out of phase.

See:
http://scienceworld.wolfram.com/physics/ConstructiveInterference.html
http://scienceworld.wolfram.com/physics/DestructiveInterference.html