Diketahui deret 4+9+14+19...+104 tentukan jumlah deret tersebut

U1 = a = 4
beda = b = 9-4 = 5
Un = a + (n - 1)b
104 = 4 + (n - 1)5
104 - 4 = 5n - 5
100 = 5n - 5
100 + 5 = 5n
105 = 5n
n = 105/5 = 21
Sn = n/2(a+Un)
S21 = 21/2(4+104)
S21 = 21/2(108)
S21 = 1134