jumlah bilangan kelipatan 9 antara 200 dan 400 ?

a = 207
b = 9
Un = 396
Un = a + ( n - 1 ) b
396 = 207 + ( n - 1 ) 9
396 = 207 + 9n - 9
-9n = 207 - 9 - 396
-9n = -198
n = 22

Sn = n/2 ( 2a + ( n - 1 ) b)
Sn = 22/2 ( 2(207) + ( 22 - 1 ) 9)
Sn = 11 ( 414 + 189 )
Sn = 11 ( 603 )
Sn = 6633