Long-run Average Cost
In Figure 1, for the output level at which a given SAC is tangent to the LAC, the long-run marginal cost (LMC) of producing that level of output is the same as the short-run marginal cost (SMC). Thus, LMC(Q1) = SMC(Q1), LMC(Q2) = SMC(Q2), and LMC(Q3) = SMC(Q3). Each point along a given SAC curve, expect for the tangency point, lies above the corresponding point on the LAC curve. The minimum point on the LAC curve in Figure 2 (Q=Q2), the long-run and short-run marginal and average costs all take exactly the same value.
Figure 1 The family of cost curves associated with a u-shaped LAC
References
The Open University of Hong Kong (1998) 'Production' in EC301 Economic Analysis of Business and Public Policies, Hong Kong: OUHK.
Pashigian, P B (1998) Price theory and application, New York: Worth.