When I saw Example 3, I was stumped by this question. The title is: "There is a pile of wood stacked together, one * * * is 20 floors, the first floor has 12, the second floor has 13, ..., and each floor below is one layer more than the previous one. How many * * * are there in this pile of wood? " I picked up a pencil and drew a picture on the left and a picture on the right in the draft book, but I didn't work out the problem either. In class, the teacher gave us a formula-trapezoidal area = (upper bottom+lower bottom) × height ÷2. What does this formula mean? Why use such a formula? I thought for a long time, but I couldn't find the answer, so I asked the teacher this question. The teacher said, "You can think about the calculation method of rectangular area." According to the teacher's prompt, I drew a picture in my notebook, calculated it and drew the following picture.
It turns out that the sum of the upper bottom and the lower bottom of a trapezoid is the length of a rectangle, the height of the trapezoid is the width of the rectangle, and the area of the trapezoid is half of the area of the rectangle, so the formula of trapezoid area = (upper bottom+lower bottom) × height ÷2 is obtained. Because this pile of wood has become a trapezoid, we can use this formula to calculate its root number, so I quickly calculated the length of the bottom (that is, how many roots are there)12+20-1= 31(roots), and then calculated the area of the trapezoid, that is, the total root number of this pile of wood, (65433. I got this question right! Yeah! " I said excitedly.
After finishing this problem, the teacher said, "Students, this kind of problem is similar to finding the trapezoidal area, which will be learned in the fifth grade." My heart is full of joy, because I have learned a fifth-grade arithmetic problem since I was only in the third grade, and I also know the origin of trapezoidal area formula and how to apply it to daily life.