等差級數答案

1 +4 + 7 ...... + 46

第一項：T(1) = a = 1
公差： d = 4 - 1 = 7 - 4 = 3

T(n) :
a + (n - 1)d = 46
1 + (n - 1) x 3 = 46
3(n - 1) = 45
n - 1 = 15
n = 16
項數 = 16

和
= 1 + 4 + 7 ...... + 46
= n[T(1) + T(n)] / 2
= 16 x (1 + 46) / 2
= 376

項數 : an= 46
= 1+(n-1)3
= 3n-2
3n=48
n= 16

n項的和 : S16
= 16x(46+1) / 2
= 8x47
= 376