✔ 最佳答案
Sn = Σan= Σn(n+1)/2= Σ(n^2 + n)/2= Σn^2/2 + Σn/2= n(n+1)(2n+1)/12 + n(n+1)/4= [n(n+1)/4]*[(2n+1)/3 + 1]= [n(n+1)/4]*[(2n+4)/3]= n(n+1)(2n+4)/12= n(n+1)(n+2)/6= answer
S15 = 15*16*17/6= 5*8*17= 17*40= 680= answer
2015-03-07 07:51:34 補充:
2.上題可以寫作: Sn = 1*(b-1) + 2*(b-2) + 3*(b-3) + ...
b = an + 1
Ans: 可以!!
Sn = Σn*(b-n)
= Σb*n - Σn^2
= b*n(n+1)/2 - n(n+1)(2n+1)/6
= [n(n+1)/2]*[b - (2n+1)/3]
= [n(n+1)/2]*(3b-2n-1)/3
= [n(n+1)/2]*[3(an+1) - 2n - 1]/3
= [n(n+1)/2]*[3(n + 1) - 2n - 1]/3
= [n(n+1)/2]*(3n + 3 - 2n - 1)/3
2015-03-07 07:52:20 補充:
= [n(n+1)/2]*(n+2)/3
= n(n+1)(n+2)/6
= 與上題答案相同