6 + 12 + 18 + 24 +........+ 36000

6 + 12 + 18 + 24 +........+ 36000 =
[(36000 - 6 + 6) x (36000 + 6)] : 2 x 6
[36000 x 36006] : 12
=108.018.000
Jadi, hasil penjumlahan dari 6 + 12 + 18 + 24 +........+ 36000 = 108.018.000

Un = U1 + (n - 1)b = 36000
     = 6 + (n - 1)6 = 36000

⇔ 6 + (n - 1)6 = 36000
⇔ 6 + 6n - 6 = 36000
⇔ 6n = 36000 - 6 + 6
⇔ 6n = 36000
⇔ n = 36000/6
⇔ n = 6000

Sn = n/2 (U1 + Un)
S₆₀₀₀ = 6000/2 (6 + 36000)
S₆₀₀₀ = 3000 x 36006
S₆₀₀₀ = 108018000