T he ACME Corporation determines that at current prices the demand for its computer chips has a price elasticity of –2 in the short run, while the price elasticity for its disk
drives is –1.
   If the corporation decides to raise the price of both products by 10 percent, what will happen to its sales? To its sales revenue?
这是平迪克微观经济学第七版上的一道习题，给出的答案是：
  Note: The answer at the end of the book (first printing) for the percent change in disk drive sales revenue is incorrect. The correct answer is given below. We know the formula for the elasticity of demand is EP = %ΔQ/%ΔP. For computer chips, EP = –2, so –2 = %ΔQ/10, and therefore %ΔQ = –20. Thus a 10 percent increase
in price will reduce the quantity sold by 20 percent. For disk drives, EP = –1, so a 10 percent increase in price will reduce sales by 10 percent。
Sales revenue will decrease for computer chips because demand is elastic and price has increased. We can estimate the change in revenue as follows. Revenue is equal to
price times quantity sold. Let TR1 = P1Q1 be revenue before the price change and TR2= P2Q2 be revenue after the price change. Therefore ΔTR = P2Q2 – P1Q1, and thus ΔTR
= (1.1P1 )(0.8Q1 ) – P1Q1 = –0.12P1Q1, or a 12 percent decline.Sales revenue for disk drives will remain unchanged because demand elasticity is –1.
  我的疑惑是：对于需求弹性为-1的disk drives 来说，当他的价格上涨10%以后，它的销售收入真的不变吗？
备注：平迪克在书中已经说明了，在没有特殊说明的情况下，弹性就是指点弹性，也就是说本题中的弹性是指点弹性，根据论坛上的这篇帖子：http://www.pinggu.org/bbs/thread-53117-1-1.html，我认为对disk drives 来说，当他的价格上涨10%以后，它的销售收入是下降的！！！
请高手多多指教！！！！！