AC curve is U-shaped in short run due to operation of law of returns to factors (i.e., law of variable proportion). Initially production is subject to law of increasing returns (i.e. decreasing cost), then law of constant return (i.e. constant cost) and ultimately to law of diminishing return (i.e. increasing cost). As output is increased, AC first falls, reaches its minimum and then rises. Hence, AC curves become U-shaped.
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AC curve is U-shaped in short run due to operation of law of returns to factors (i.e., law of variable proportion). Initially production is subject to law of increasing returns (i.e. decreasing cost), then law of constant return (i.e. constant cost) and ultimately to law of diminishing return (i.e. increasing cost). As output is increased, AC first falls, reaches its minimum and then rises. Hence, AC curves become U-shaped.