Answer:
Explanation:
(2n+1)(n+5) - 2(n+3) - (5n+13) = 2n² + 4n - 14 (*)
+) if n is divisible by 3 ⇒ n= 3k ⇒ (*)⇔ 2.9k² + 4.9k -14
2.9k² + 4.9k = 3( 2.3k² +4.3k) ║ 3 but 14∦3
⇒ with n =3k, (*)∦ 3 (1)
+) if n = 3k + m (m∦ 3)
⇒ (*) ⇔ 2.(3k+m)² + 4.(3k+m) -14 = 2.9k² + 2.3k.m + 2m² + 4.3k + 4m -14
= 3(2.3k² + 2.km + 4k) +2m² + 4m -14
3(2.3k² + 2km + 4k)║3 but (2m²+4m-14)∦3
⇒ with n = 3k+m, (*) ∦ 3 (2)
(1)(2)⇒ [(2n+1)(n + 5) - 2(n+3) - (5n+13)] ∦ 3
⇒ (2n+1)(n+5)–2(n+3)–(5n+13) is not divisible by 6 for any whole n
- ║is divisible
- ∦ is not divisible