The marginal utility of X is the additional utility from one additional unit of X or, more formally, the derivative of utility with respect to X. The marginal utility of X may very well depend on the amounts of other goods, Y and Z, for example, that also enter a utility function.
Marginal utility is an important concept because the theory of the consumer generally assumes that agents consider marginal utilities when deciding how much of X (and Y and Z) to consume. Marginal utility plays an important role in the Theory of the Consumer, which includes the EconModel applications Two Goods - Two Prices, Labor Supply, Income Taxes, and Transfer Payments , and Intertemporal Substitution.
Classic Economic Models
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Balance of Payments
Endogenous Technical Change
Federal Funds (Fed Funds) Rate
Fixed Exchange Rate
Floating Exchange Rate
Gross Domestic Product (GDP)
Production Possibility Frontier
Reservation Wage Rate
Theory of the Consumer
Theory of the Firm
Velocity of Money