dari deret aritmatika 10+9+8 tentukan jumlah 20 suku pertama

a = u₁
= 10
b = u₂ - u₁
= 9 - 10
= -1

sₙ = n/2 × {2a + (n - 1)b}
s₂₀ = 20/2 × {2(10) + (20 - 1)(-1)}
s₂₀ = 20/2 × {2(10) + (19)(-1)}
s₂₀ = 10 × {20 + (-19)}
s₂₀ = 10 × 1
s₂₀ = 10