suatu barisan aritmetika memiliki suku kedua 8, suku keempat 14, suku terakhir 23. jumlah semua suku barisan

U2 = a + (2-1)b
U2 = a + b = 8 …(1)

U4 = a + (4-1)b
U4 = a + 3b = 14 …(2)

Persamaan (1) dan (2)
a + b = 8
a + 3b = 14 (-)
-2b = -6
b = 3

b = 3
a + b = 8
a + 3 = 8
a = 5

Un = a + (n-1)b
23 = 5 + (n-1)3
23-5 = 3n-3
18 = 3n-3
3n-3 = 18
3n = 18+3
3n = 21
n = 7

S7 = 7/2 ( a + U7 )
S7 = 3.5 ( 5 + 23 )
S7 = 3.5 ( 28 )
S7 = 98