This document discusses two-sector macroeconomic equilibrium using the consumption-investment (C+I) model and the savings-investment (S=I) model. It provides the mathematical formulas and graphs for determining equilibrium output and explains how shocks to planned spending, savings, or investment can cause the economy to contract or expand towards a new equilibrium through adjustment mechanisms. Examples are given to illustrate calculating equilibrium GDP, the effect of changes in investment on GDP, and using the investment multiplier to determine the change in GDP or required change in investment given an exogenous change in output.