Meaning: a1=12-8×1=-7.
According to the condition sn = n 2-8n ...
s(n- 1)=(n- 1)^2-8(n- 1)…②
①-②: sn-s(n- 1)=2n-9。
By an=sn-s(n- 1)
So an=2n-9, and this formula applies to a 1.
So the general formula of {an} is 2n-9.
N is an integer, 2n-9 < 0 when n≤4, and 2n-9 > 0 when n≥5.
Therefore, the general formula of {|an|}
When n≤4, |an|=9-2n.
When n≥5, | an | = 2n-9;
(2)
When n≤4
Every term is negative, so add a negative sign to remove the absolute value.
So pn = 8n-n 2 (n ≤ 4).
When n≥5,
Because S4 = a1+A2+A3+A4 = |-7 |+|-5 |+|-kloc-0/| =16.
Therefore, pn = S4+[1+3+...+(2n-9)] = (1+2n-9) (n-4)/2+16 = n 2-8n+32.
Therefore, when n≤4, pn = 8n-n 2.
When n≥5, pn = n 2-8n+32.