why should we take some angles on the outer side of the protracter and some of them in the inner side to contruct the angles in a triangle??please do answer fast ...my exam is day after

Since we measure angles in clockwise manner you are supposed to use the inner measures. You an use the outer readings as well but then you'll have to make sure you know where exactly you want your zero to be. What this means is that the base arm from which the angle starts must coincide with zero.