清华出的 英文影印版的微观经济学（平迪克,第六版）

Chapter 4, section 4.3, elasticity of demand 这一部分 （p.124)
讲了 Inelastic demand, elastic demand, unit elastic demand 三种情况

关于 Inelastic demand部分， 讲到 when demand is inelastic, the quantity demanded is relatively unresponsive to changes in price. As a result, total expenditure on the product increases when the price increases.  然后书上给了个例子，suppose, for example, that a family currently uses 1000 gallons of gasoline a year when the price is $1 per gallon; suppose also that our family's price elasticity of demand for gasoline is -0.5. If the price of gasoline increases to $1.10 (a 10-percent increase), the consumption of gasoline falls to 950 gallons (a 5-percent decrease). Total expenditure on gasoline, however, will increase from $ 1000 (1000 gallons * $1 per gallon) to $ 1045 (950 gallons * $1.10 per gallon).

我的问题是如上的解释对吗？它直接用 Elasticity = Q% / P% （不会输入delta符号） 这样解释准确吗？ 我认为应该用 Arc elasticity的概念解释，那样算出来新的Quantity 是 953， 虽然数值差别不大， 但我不太明白书上的方法。请知道的人帮我解答下吧，谢谢了

同样， 在解释unit elasticity时，书上的例子说 当Price Elasticity of demand = -1时，Total expenditure remains the same after a price change. suppose the total expenditure on movies in Berkeley, is $5.4 million per year. If the price is $6, the quantity will be 900000 tickets; if the price increases to $9, the quantity will drop to 600000 tickets.

只有用 Arc elasticity来算才能得出例子中所述的结论

谢谢大家帮忙