who came up with the law of diminishing marginal utility
Marginal utility Carl Menger presented the theory in Grundsätze der Volkswirtschaftslehre (translated as Principles of Economics) in 1871. Menger's presentation is peculiarly notable on two points. First, he took special pains to explain why individuals should be expected to rank possible uses and then to use marginal utility to decide amongst trade-offs. (For this reason, Menger and his followers are sometimes called “the Psychological School”, though they are more frequently known as “the Austrian School” or as “the Vienna School”.) Second, while his illustrative examples present utility as quantified, his essential assumptions do not.[11] (Menger in fact crossed-out the numerical tables in his own copy of the published Grundsätze.[39]) Menger also developed the law of diminishing marginal utility.[14] Menger's work found a significant and appreciative audience.
Marginal revenue For a firm facing perfect competition, price does not change with quantity sold (   P ′ ( q ) = 0  {\displaystyle P'(q)=0} ), so marginal revenue is equal to price. For a monopoly, the price decreases with quantity sold (   P ′ ( q ) < 0  {\displaystyle P'(q)<0} ), so marginal revenue is less than price (for positive   q  {\displaystyle q} ).