Question

(b) Given the demand function

quantity demanded = 650-5P-P^2

where P=10

determine the price elasticity of demand.

Answer 1

Elasticity is the slope (derivative) of the demand function. 

Qd = 650 - 5P - P^2 
Qd' = -5 - 2P 
Where P = 10, we get 
Qd' = -5 - 2*10 = -25

Answer 2

Demand is the act of how supply of goods in the market is being done sometimes demand is high and at some points demand is low.So price elasticity measure the responsiveness of demand,this is after change in a good own price.The elasticity depend on the the rate at which change in demand with respect to change in price is in the market.The solution given below.