質問: 20 p
回答: given: p(n, 5) = 20 p(n, 3)to find: value of nwe know,so, according to question:p(n, 5) = 20 p(n, 3)⇒ (n – 3) (n – 4) = 20⇒ n2 – 3n – 4n + 12 = 20⇒ n2 – 7n + 12 – 20 = 0⇒ n2 – 7n – 8 = 0⇒ n2 – 8n + n – 8 = 0⇒ n(n – 8) – 1(n – 8) = 0⇒ (n – 8) (n – 1) = 0⇒ n = 8 or 1for, p(n, r): n ≥ r∴ n = 8 {for, p(n, 5)}