WHY is it that in some questions of relative motion we use pythagorus theorem while in SOME OTERS we just simply add the vel.s ??

If the two velocities act in one dimension only, then we either add/subtract the two to get the relative velocity. And if the two velocities act perpendicular to each other (i.e. in a plane), we use pythagorus theorem to find the relative velocity.

 

However, the general formula for the relative velocity is