Analysis: The serial number of this number is 1 2 3...n
Numbers a a+d a+d+d...a+(n- 1)*d
S=n*a+a+(n- 1)*d/2 (①)
Two conditions, that is, the annual planting area (mu) of seedlings is 4000 5000 6000, and the annual selling area (mu) of seedlings is 2000 2500 3000, can be withdrawn:
Annual net increase area (mu): 2000-2500-3000.
In the above series: a=2000 d=500 (②)
It is known that there were 20,000 mu of seedlings in this town at the end of 2000. Therefore, when the sum of the annual increase area is S = 30,000 mu (③), the value of n is the year.
Combining the formula of ①, ② and ③, we can find that when n=8 and S=30000,
So eight years later, in 2008, the area of seedlings in this town reached 50,000 mu.
Off-topic: I learned the old version before the new curriculum reform, and the specific method of formula ① here was learned in high school. I don't know if it is mentioned in the junior high school textbook after the new curriculum reform, but I can't remember the specific details. In short, this series is called arithmetic progression, that is, the sum of the two numbers before and after is exactly twice that in the middle.