long run average cost curve (LRAC)
known as the 'envelope curve
points of tangency between the LRAC and SRAC curves do not occur at the minimum points of the SRAC curves except at the point where minimum efficient scale (MES) is achieved
if LRAS is falling when output is increasing then the firm is experiencing economies of scale
when LRAC eventually starts to rise then the firm experiences diseconomies of scale, and if LRAC is constant then the firm is experiencing constant returns to scale
working assumption that a business will choose the least-cost method of production in the LR