有些应用题,按题目的条件顺序去思索探求解答方法比较困难,有时,还会出现繁杂的运算。如果能逆向推理,将题目的条件顺序颠倒过来去思考,解答起来往往会方便许多。 例1.王老师上街买书,第一次用去所带钱的一半,并从银行取出36.80元;第二次用去身边所有钱的一半还多12.70元,此时,还剩下30元。王老师原来有钱多少元? 【思路】第二次买书前王老师身边有钱:(30+12.70)×2=85.40(元);第一次买书前王老师身边有钱:(85.40-36.80)×2=97.20(元)。 解:[(30+12.7)×2-36.8]×2=97.20(元) 答:王老师原来有钱97.20元。 2.小明看一本科技书,第一天看了全书的1/2还多2页,第二天看了余下的1/2还少1页,剩下20页没有看完。问小明第一天看书多少页? 【思路】根据已知条件,利用“逆推法”倒着 Some application questions, according to the order of the conditions of the order to explore the search method is more difficult, sometimes, there will be complex calculations. If you can reverse reasoning, the order of the conditions of the order upside down to think, it will often be easier to answer many. Example 1. Mr. Wang went to the streets to buy a book, for the first time spent half of the money, and removed from the bank 36.80 yuan; the second time spent more than half of all the money more than 12.70 yuan, this time, leaving 30 yuan . Wang how much the original money? [Ideas] Wang bought the book before the money around the second: (30 + 12.70) × 2 = 85.40 (million); the first time before buying a book teacher Wang money: (85.40-36.80 ) × 2 = 97.20 (yuan). Solution: [(30 + 12.7) × 2-36.8] × 2 = 97.20 (yuan) Answer: Mr. Wang originally had a richness of 97.20 yuan. 2. Xiao Ming read a science and technology book, the first day saw the book more than 1/2 more than the next day saw the remaining 1/2 less 1 page, the remaining 20 pages did not finish. Asked how many pages Xiaoming read the first day? [Ideas] According to known conditions, the use of “reverse method” down