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s₁₀ = -125
s₂₀ = -750
Rumus Sn = n/2 (2a+(n-1)b)
S₁₀ = 10/2 (2a+9b) = -125
10a+45b = -125
10a = -125-45b
S₂₀ = 20/2 (2a+19b) = -750
20a+190b = -750 (kedua sisi dibagi 2)
10a + 95b = -375
10a = -375-95b
gabungkan kedua persamaan
-125-45b = -375-95b
-45b+95b = -375+125
50b = -250
b = -250/50
b = -5
10a = -125-45b
10a = -125-(45.-5)
10a = -125+225
10a = 100
a = 100/10 = 10
Un = a + (n-1)b
U68= 10 + (68-1).-5 = 10 + 67.-5 = 10 - 335 = -325 (C)