(2007 Guangzhou level test) the sum of the first n terms of the arithmetic sequence {an} is known as Sn, A2 = 2, S5 = 0. (1) find the general formula of the sequence {an}; (2) when n is the value, Sn gets the maximum

(2007 Guangzhou level test) the sum of the first n terms of the arithmetic sequence {an} is known as Sn, A2 = 2, S5 = 0. (1) find the general formula of the sequence {an}; (2) when n is the value, Sn gets the maximum

(1) ∵ A2 = 2, S5 = 0, ∵ a1 + D = 25a1 + 5 × 4D2 = 0, the solution is A1 = 4, d = - 2. ∵ an = 4 + (n-1) × (- 2) = 6-2n. (2) Sn = Na1 + n (n − 1) D2 = 4N − n (n − 1) = - N2 + 5N = − (n − 52) 2 + & nbsp; 254. ∵ n ∈ n *, ∵ when n = 2 or n = 3, the maximum value of Sn is 6

Let Sn be the sum of the first n terms of the arithmetic sequence {an}, if a5a3 = 59, then s9s5 = ()
A. 1B. -1C. 2D. 12

Let the first term of the arithmetic sequence {an} be A1. From the properties of the arithmetic sequence, we can get a1 + A9 = 2a5, a1 + A5 = 2A3, ｜ s9s5 = a1 + a92 × 9a1 + a52 × 5 = 9a55, A3 = 95 × 59 = 1, so we choose a

Let Sn be the first n term of the arithmetic sequence and A5 / A3 = 5 / 9, then S9 / S5=

S9 = 9A5
S5 = 5A3
So S9 / S5 = 9A5 / 5A3 = 1

Let Sn be the sum of terms of the arithmetic sequence (an). If A5 / A3 = 5 / 9, then S9 / S5 is equal to?

If the difference is equal, a1 + A9 = 2a5
a1+a5=2a3
S9/S5
=[(a1+a9)*9/2]/[(a1+a5)*5/2]
=(2a5*9/2)/(2a3*5/2)
=9a5/5a3
=(9/5)*5/9
=1