Jumlah bilangan ganjil 9 sampai dengan 213

9 + 11 + 13 + ... + 213

a = 9
b = 2
Un = 213

Un = a + (n-1)b
213 = 9 + (n-1)2
213 = 9 + 2n - 2
213 = 2n + 7
2n = 213 - 7
2n = 206
n = 206 / 2
n = 103

Sn = n/2(a + Un)
S103 = 103/2 (9 + 213)
S103 = 103/2 x 222
S103 = 103 x 111
S103 = 11.433