Given that A1 = 9, A3 + A8 = 0 in the arithmetic sequence {a n}, find 1. The general term formula of the sequence {a n} 2. When n is the value, the first few terms of the number weight {a} and Sn take the maximum And find the maximum value

Given that A1 = 9, A3 + A8 = 0 in the arithmetic sequence {a n}, find 1. The general term formula of the sequence {a n} 2. When n is the value, the first few terms of the number weight {a} and Sn take the maximum And find the maximum value

a1=9
a3+a8=0
a1+2d+a1+7d=0
2a1+9d=0
18+9d=0
d=-2
an=9+[-2(n-1)]
an=11-2n
When SN is negative, it doesn't work, so A5 = 1
So we can only go to the first five terms, Sn = 25

Write a general term formula of 1 / 3, 1, 9 / 5, 8 / 3, and 1, 0, - 1 / 3, 0, 1 / 5, 0, - 1 / 7, 0

n²/(n+2)
cos(nπ/2) / n

Fill in the blanks with ten numbers 0123456789. Each number can only be used once

0＋9＝1＋8＝2＋7＝3＋6＝4＋5

With the five numbers of 0, 1, 2, 3 and 4, the decimal point "." and the multiplication sign "X", which decimal multiplication formula can you write? And calculate

0.1*0.1
0.1*0.2
0.1*0.3
0.1*0.4
.
10