The demand curve q is a function of the price p, and is denoted as Q(P). Total income curve TR= sales quantity Q * sales price p, which is the total market demand. Q = Q brings the demand curve into the total income curve, and the total income curve TR is a function of price, which is denoted as TR(P). The so-called total income curve comes from demand curve and price to some extent.

Marginal income curve is the derivative of total income curve to price (or quantity), so marginal income curve also comes from demand curve.

Marginal income curve:

Why does MR decline faster than the demand curve in monopoly enterprises? Let's use a mathematical formula first: suppose the demand curve is P=aQ+c(Q demand, P price), then R(R income) = QP = AQ2+CQ；; The derivative of q can be obtained: MR=( 1/2)aQ+c, which is the relationship between marginal revenue MR and q; Obviously, the slope of MR formula is 1/2 of the slope of demand curve.

So MR is naturally lower than the demand curve. Answer in words: If monopoly enterprises can adopt ladder pricing, that is to say, the first product is sold at one price and the second product is sold at one price, then the demand curve is not only a marginal income curve, but in most cases, consumers buy a variety of goods at a unified price.

No one wants to pay with the ladder price, so the unified price is the pricing of the last unit demand, which is why the marginal income curve is lower than the demand curve. Actually, I think it's clearer to use formulas.