Constructive interference and destructive interference

[gracy]

Is he right there?

Is there a principle that says "For every constructive interference, there is an equal (but opposite) destructive interference?" Is it impossible for constructive interference to occur without any destructive interference?I don't think so.Because when there is a constructive interference there is all brightness,especially when sources are coherent.
Here the two waves are inphase.And constructive interference are happening there ,how can there be any destructive interference?
http://www.antonine-education.co.uk/Salters/MUS/images/Making3.jpg

 

[PWiz]

If the two waves are moving in opposite directions, then after $\frac{T}{4}$ seconds, can you guess where their respective crests and troughs be positioned? ($T$=Time period of the waves)

 

[sophiecentaur]

Here the two waves are inphase.And constructive interference are happening there ,how can there be any destructive interference?

The situation you have drawn (a maximum) refers to a point in space where the waves are in phase. But the relative phases are not the same everywhere. If you move a quarter of a wavelength towards one of the sources, the wave from it will be 90 degrees advanced but the wave from the other source will be 90 degrees late - 180 degrees difference - so you will get subtraction (a minimum).
Sorry - I should probably have made you think more.