问题如下：
The demand for membership at a local health club is determined by the following equation:
Qdhm = − 400 − 5 Phm
where Qdhm is the number of health club members and Phm is the price of membership. If the price of health club membership is $35, the price elasticity of demand is closest to:
选项：
A.–0.778.
B.–0.500.
C.–0.438.
解释：
A is correct.
Inserting the price of $35 into the demand function, quantity demanded is calculated as
Qdhm = − 400 − 5 (35) =225
At a price of $35 per health club membership, the elasticity of demand is Price elasticity of demand = (∆Qdhm / ∆Phm)× ( Phm/Qdhm)
Price elasticity of demand = –5 × (35/225) = –0.778
考点：点弹性
解析：
当价格等于35的时候，依据需求函数
Qdhm = − 400 − 5 Phm和点弹性公式(∆Qdhm / ∆Phm)× ( Phm/Qdhm)可得点弹性数值：
– 5 × (35/225) = – 0.778
题目写错了，400前不应该有负号
