The product of 1- 100 is equal to 9.332622e 157, which is beyond the learning scope of junior high school.

The sum of 1-n is equal to (1+n)n/2.

The product of 1-n cannot be expressed by a general term, but can only be substituted into a specific numerical calculation.

In the difference arithmetic progression, the sum of two terms with the same distance as the first two terms is equal. And is equal to the sum of the first two terms and the last two terms; In particular, if the number of items is odd, it is equal to twice the number of items in the middle. The arithmetic mean term is half of the sum of arithmetic progression's head and tail terms, but you don't have to know the head and tail terms to find the arithmetic mean term.

For example: 1, 3, 5, 7, 9...2n- 1. The general formula is: an = a1+(n-1) * D. The first term a 1= 1, and the tolerance d=2. The first n terms and formulas are: sn = a1* n+[n * (n-1) * d]/2 or Sn=[n*(a 1+an)]/2. Note: All the above n are positive integers.

Extended data:

Arithmetic progression's other inferences:

① Sum = (first item+last item) × number of items ÷2.

② Number of items = (last item-first item) ÷ tolerance+1.

③ The first term =2x and the number of terms-the last term or the last term-tolerance × (the number of terms-1).

④ The last item =2x and the number of items-the first item.

⑤ The last term = the first term+(number of terms-1)× tolerance.

⑥2 (sum of the first 2n terms and-the first n terms) = sum of the first n terms and+the first 3n terms and-the first 2n terms.

Baidu Encyclopedia-arithmetic progression