2 Q2 |link| — Hkcee 2010 Econ Paper

Alternatively, geometric: PS_new = rectangle (0 to 10, price 80 to supply at Q=10? Wait, need triangle + rectangle). Better: PS = area above supply and below price. At Q=10, supply price = ( 20+30=50 ). So PS = rectangle (10 × (80-50)) + triangle beneath that? No – actually supply is linear: PS = area between P=80 and supply from Q=0 to 10 = trapezoid: average height = (80-20 + 80-50)/2 = (60+30)/2 = 45; area = 45 × 10 = 450. Yes.

Compute PS as area under price $80 down to supply curve, from Q=0 to Q=10 (quantity sold). That’s a trapezoid? Actually simpler: PS = [price × quantity sold] – area under supply curve from 0 to 10. hkcee 2010 econ paper 2 q2

From demand: ( 80 = 100 - 2Q_d \implies 2Q_d = 20 \implies Q_d = 10 ) tonnes. From supply: ( 80 = 20 + 3Q_s \implies 3Q_s = 60 \implies Q_s = 20 ) tonnes. Alternatively, geometric: PS_new = rectangle (0 to 10,

Area under supply from 0 to 10: Supply P=20+3Q, integral = ( 20Q + 1.5Q^2 ) evaluated 0 to 10 = ( 200 + 150 = 350 ). Revenue = ( 80 \times 10 = 800 ). So PS_new = ( 800 - 350 = 450 ). At Q=10, supply price = ( 20+30=50 )