5. Suppose that the demand curve for compact disks is given by P = 600 - Q and that the supply curve is given by P = 0.5Q, where Q is the quantity of compact disks and P is their price. What is the price elasticity of demand at the equilibrium price and

quantity?

(I understand everything except why we use the derivative of p= 600-Q to get -1 instead of p=0.5q for the own price elasticity formula. If you can explain this it would be great. Thanks.

A)-0.05

B)-0.02

C)-0.20

D)-0.50(correct)

E)-2.00