Sn=1/2n(5n-19). Rumus suku ke n barisan tersebut adalah

dianggap gini → Sn = (1/2)n(5n - 19)
S1 = (1/2)1(5 - 19) = -7
S2 = (1/2)2(10 - 19) = -9
Un = Sn - S(n-1)
U2 = U2 - U1 = -9 - (-7) = -2
U1 = S1 = a = -7
b = U2 - U1 = -7 - (-2) = -5
Un = a + (n - 1)b = -7 + (n - 1)(-5)
Un = -2 - 5n