Bila jumlah n suku pertama daret aritmatika ditentukan oleh formula Sn = 2n² + 8n. suku ke-8 deret itu sama dengan

Sn = 2n² + 8n
S1 = 2(1)² + 8(1) = 10
S2 = 2(2)² + 8(2) = 8 + 16 = 24

S1 = U1 = a
S2 = U1 + U2
24 = a + a + b
24 = 2a + b
24 = 2(10) + b
24 = 20 + b
b = 24 - 20
b = 4

U8 = a + 7b
= 10 + 7(4)
= 10 + 28
= 38

Sn = 2n^2 +8n.
S1= a = 2(1)^2 +8(1) = 2+8= 10
S2 = U1+ U2 = a + (a+b) = 2a +b
2a + b = 2(2)^2 +8(2) = 8+16= 24
jadi 2a+ b = 24. subsitusi a = 10
2.10 + b = 24
20 + b = 24
b = 24-20
b = 4
Un = a + (n-1)b
U8 = a + 7b
= 10 + 7.4
= 10 + 28
= 38
Jadi U8 = 38