为什么这题用的是-3.1。而不是0.9

问题如下：

The market demand function for four-year private universities is given by the equation

Q^d_pr ​=84−3.1P_pr​+0.8I+0.9P_pu​

whereQ^d_pr is the number of applicants to private universities per year in thousands, P_pr​is the average price of private universities (in thousands of USD), I is the household monthly income (in thousands of USD), and P_pu​ is the average price of public (government-supported) universities (in thousands of USD). Assume that P_pr​ is equal to 38, I is equal to 100, and P_pu​ is equal to 18.

The price elasticity of demand for private universities is closest to:

选项：

A.

–3.1.

B.

–1.9.

C.

0.6.

解释：

B is correct.

From the demand function:

ΔQ^d_pr/ΔP_pr​=−3.1

Solve for Q^d_pr = 8r−3.1P_pr​+0.8I+0.9P_pu

=84−3.1(38)+0.8(100)+0.9(18)=62.4

At P_pr​ = 38, price elasticity of demand=(ΔQ^d_pr​/ΔP_pr)(P_pr​/Q^d_pr​)​=(-3.1)(38/62.4)=-1.9

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，先求的私立大学的需求量，即：

Q^d_pr = 8r−3.1P_pr​+0.8I+0.9P_pu

=84−3.1(38)+0.8(100)+0.9(18)=62.4

之后我们再利用需求价格弹性公示求解，即，

(ΔQ^d_pr​/ΔP_pr)(P_pr​/Q^d_pr​)​=(-3.1)(38/62.4)=-1.9