Summation

答案：13

你地攪咩呀，而家 i 係變數喎，n 係常數。
即係每個項都係 n，問題係有幾多個項0者……
由正整數 6 到正整數 n，總共有 n - 6 + 1 咁多個項。
所以個總和就係 (n - 6 + 1) 咁多個 n 囉。

即係
(n - 6 + 1)n = 104
(n - 5)n = 104
n^2 - 5n - 104 = 0
n = 13 (捨去 n = -8)

n n 5
Σn=Σn-Σn=n(n+1)/2 - 5(5+1)/2=104
6 1 1
n^2+n-30=208
n^2+n-238=0
n= -1±√[1-4(1)(-238)] / 2
= -0.5± √953
n= -0.5+√953/2 or -0.5-√953/2
n must be positive
∴n= -0.5+√953/2 =14.93534904...//

(1+n)n/2 - (1+5)5/2 = 104
n+n^2 - 30 = 208
n^2+n-238 = 0
=> n is not an integer.

Σn from i = 6 to i = 15 is 105.

∑ (6->n) n = 104

n(n - 5) = 104

n^2 - 5n - 104 = 0

(n - 13)(n + 8) = 0

n = 13