请问为啥不能选-5，这道题给的公式似乎也没有其他变量，到底什么时候可以直接用-5什么时候用点弹性去算呢？

问题如下：

The demand for membership at a local health club is determined by the following equation:

Q^d_hm = 400 − 5 P_hm

where Q^d_hm is the number of health club members and P_hm is the price of membership. If the price of health club membership is $35, the price elasticity of demand is closest to:

选项：

–0.778.

–0.500.

–0.438.

解释：

A is correct.

Inserting the price of $35 into the demand function, quantity demanded is calculated as

Q^d_hm = 400 − 5 (35) =225

At a price of $35 per health club membership, the elasticity of demand is Price elasticity of demand = (∆Q^d_hm / ∆P_hm)× ( P_hm/Q^d_hm)

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

Q^d_hm = 400 − 5 P_hm和点弹性公式(∆Q^d_hm / ∆P_hm)× ( P_hm/Q^d_hm)可得点弹性数值：

– 5 × (35/225) = – 0.778