【问题】有三个数3、2.5、1.5,再找一个数,使这四个数能组成比例。想一想:能写出几个不同的比例式?【思路点睛】怎样快速找到一个数与3、2.5、1.5组成比例呢?我们不妨设这个数为x,根据比例的意义,x必须与3、2.5、1.5这三个数中的某一个数同时作为外项或内项,剩下的两个数作为内项或外项。先把3与x结合作为外项,写出比例式3∶2.5=1.5∶x,求出x=2.5×1.5÷3=1.25,将1.25代入到上面的式子中,得到3∶2.5=1.5∶1.25。以这个比例式为基础,交换内项、外项的位置,可以得到三个新的比例式。 [Question] There are three numbers 3, 2.5, 1.5, then find a number, so that these four numbers can form a ratio. Think about it: Can you write a few different ratios? How do I quickly find a number with a ratio of 3, 2.5, 1.5? Let's say that this number is x, and x must be proportional to 3 , 2.5,1.5 The number of these three numbers at the same time as an external or internal items, the remaining two as the internal or external items. Combine 3 with x as the first term, write the proportional equation 3: 2.5 = 1.5: x, find x = 2.5 × 1.5 ÷ 3 = 1.25 and substitute 1.25 into the above equation to obtain 3: 2.5 = 1.5 : 1.25. Based on this proportional formula, three new proportional formulas can be obtained by exchanging the positions of the inner and outer terms.