ΔQdpf/ΔP不应该等于-1/3吗?

问题如下：

We observe the demand function for food is given by the equation

Q^d_pf ​=80−3P_pf​+0.1I+0.5P_pd​

Q^d_pf is the number of food, P_pf​ is the price of food, I is the income , P_pd is the price of drink

We know that:P_pf​= 30, I=110, and P_pd​= 20.

The price elasticity of demand for food will be:

选项：

–10.

–8.2.

0.6.

解释：

B is correct.

From above function, we get:

ΔQ^d_pf/ΔP_pf​=−3

Q^d_pf = 80−3∗30+0.1∗110+0.5∗20=11

price elasticity of demand= (-3)(30/11)= -8.2

考点：需求价格弹性

解析：题目给出了需求函数的表达式，带入数字，先求出需求量，即：

Q^d_pf = 80−3∗30+0.1∗110+0.5∗20=11

之后我们再利用需求价格弹性公示求解，即

(−3)(30/11)=−8.2