tentukan Suku ke 80 dan jumlah 80 suku dari barisan aritmatika 120,114,108,102,....

120,114,108,102, ...
a = 120
b = U2 - U1
   = 114 - 120
   = - 6

U80 = a + 79b
       = 120 + 79(-6)
       = 120 - 474
       = - 354

S80 = 80/2 (120 + (-354))
       = 40 ( - 234 )
       = - 9.360

Pembahasan:

a = u₁
= 120

b = uₙ - uₙ ₋ ₁
= u₃ - u₃ ₋ ₁
= u₃ - u₂
= 108 - 114
= -6

uₙ = a + (n - 1)b
u₈₀ = 120 + (80 - 1)(-6)
u₈₀ = 120 + 79(-6)
u₈₀ = 120 + (-474)
u₈₀ = -354

sₙ = n/2 × (a + uₙ)
s₈₀ = 80/2 × (120 + (-354))
s₈₀ = 40 × -234
s₈₀ = -9.360