long run costs and returns to scale

Question 1

what is the long run

Answer 1

all factors of production are variable

Question 2

long run average cost curve and why

Answer 2

-smiley parabola
-part one: benefitting from increasing returns to scale (percentage change of output is greater than input)
-part two: constant returns to scale, minimum efficient scale: lowest level of output to exploit all economies of scale
-part three: decreasing returns to scale (percentage change of output is less than percentage change in output)