dari barisan aritmetika diketahui suku ke 27=81 dan suku ke 49=147 jumlah 18 suku pertama adalah

Bab Pola Bilangan

U27 = a + (27 - 1)b
a + 26b = 81

U49 = a + (49 - 1)b
a + 48b = 147

a + 48b = 147
a + 26b = 81
___________ -
22b = 66
b = 3

a + 26(3) = 81
a = 81 - 78
a = 3

Sn = n/2 (2a + (n - 1)b)
S18 = 18/2 (2(3) + (18 - 1)3)
S18 = 9(6 + 51)
S18 = 9(57)
S18 = 513

U27 = 81 => a+26b = 81
U49 = 147 => a+48b =147

a+26b = 81
a+48b =147
---------------- –
-22b = -66
b=3

a+26b =81
a+78= 81
a = 3

Sn=n/2(2a+(n-1)b)
S18 = 18/2(2(3)+(18-1)3)
S18 = 9(57)
S18 = 513