NO.PZ2016010801000012
问题如下：
Below is the demand function of mobile phones:
QDmobile=10,000−2Pmobile+0.05income+2Pfixed−linetelephone−5Pwireless
At present, the price of a mobile phone is 3,000 RMB, while a fixed-line telephone and wireless are priced 1500 RMB and 500 RMB respectively. Assume income is 10,000 RMB, the corresponding income elasticity of demand for a mobile phone is:
选项：
A. 0.01.
B. 0.1.
C. 1.
解释：
B is correct.
Quantity demanded of mobile phone is QDmobile=10,000-2(3,000)+0.05(10,000)+2(1500)-5(500)=5000.
ΔQ/ ΔI=slope of income=0.05, therefore income elasticity=(I/Q) (ΔQ/ ΔI)=(10,000/5,000)0.05=0.1.
考点：income elasticity
解析：首先，依据需求函数求得MOBILE的需求量10,000-2(3,000)+0.05(10,000)+2(1500)-5(500)=5000.
此外，求得依据函数式求得ΔQ/ ΔI=0.05，
最后，把数字直接带入公式得：(I/Q) (ΔQ/ ΔI)=(10,000/5,000)0.05=0.1.
斜率的公式是怎么算出来的
