Obsen yang nomor lima b

(n + 2)!/n! = 42
(n + 2)(n + 1)(n!)/n! = 42
(n + 2)(n + 1) = 42
n^2 + n + 2n + 2 = 42
n^2 + 3n + 2 – 42 = 0
n^2 + 3n – 40 = 0
n^2 + 8n – 5n – 40 = 0
n(n + 8) – 5(n + 8) = 0
(n + 8)(n – 5) = 0
n = -8 atau n = 5 (memenuhi)