Question

Explain why 7 × 11 × 13 + 13 and 7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 are composite numbers.

Answer 1

Given numbers are 7 × 11 × 13 

7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 

⇒ 13(7 × 11 + 1) and 

5(7 × 6 × 4 × 3 × 2 × 1 + 1) 

⇒ 13 K and 5 L, where K = 78 and L = 7 × 6 × 4 × 3 × 2 × 1 + 1 = 1009 

As the given numbers can be written as product of two numbers, they are composite.