For a straight-line downward-sloping demand curve, the price elasticity of demand at the midpoint of the curve is:

解說

On a linear demand curve, slope is constant but elasticity varies. At the midpoint, price and quantity are such that the percentage changes are equal, giving PED = 1 (unitary). Above the midpoint, demand is elastic; below, it is inelastic. Zero occurs at the quantity intercept (where price is zero).