Which of the following straight-line supply curves does NOT have a price elasticity of supply equal to 1 at every point (q = quantity supplied, p = price)?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → q = 2p − 10
A straight-line supply curve through the origin has eS = 1 at every point. A line that starts on the price axis at a positive price has eS greater than 1.
Option (1) q = 5p — True. At p = 0, q = 0, so the line passes through the origin.
Option (2) q = 0.5p — True. This line also passes through the origin.
Option (3) q = 2p — True. This line also passes through the origin.
Option (4) q = 2p − 10 — False. Here q = 0 at p = 5, so the line cuts the price axis at a positive price. Its elasticity is greater than 1 at every point. Extended, the line meets the quantity axis at M = −10. At p = 10, q0 = 10, so Mq0 = 20 and eS = 20/10 = 2.
NCERT (2026-27): “The Geometric Method Consider the Figure 4.14. Panel (a) shows a straight line supply curve. S is a point on the supply curve. It cuts the price-axis at its positive range and as we extend the straight line, it cuts the quantity-axis at M which is at its negative range. The price elasticity of this supply curve at the point S is given by the ratio, Mq0/Oq0. For any point S on such a supply curve, we see that Mq0 > Oq0. The elasticity at any point on such a supply curve, therefore, will be greater than 1.” “The Geometric Method … Now we come to panel (b). Here the supply curve goes through the origin. One can imagine that the point M has coincided with the origin here, i.e., Mq0 has become equal to Oq0. The price elasticity of this supply curve at the point S is given by the ratio, Oq0/Oq0 which is equal to 1. At any point on a straight line, supply curve going through the origin price elasticity will be one.”
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