In short,the AR of the monopolist declines because a lower price is received on the additional goods sold. The MR declines at a faster rate than AR because,in selling a larger number of units,the monopolist also takes a lower price on the units that could have been sold anyway at higher price. Thus,if 3 units are sold for $8 each instead of 2 units for $9 each,total revenue increases by $8 from the sale of the third unit as such,minus $1 less on each of the two previous units that could have been sold for $9 apiece.