Question

Show that 5×7×11-121×4 is a composite number.​

Answer 1

Answer:

5 x 7 x 11 + 13

=[5 x 7 x (10+1)] +13

=[5 x 7 x 10] + [5 x 7] + 13

=[5 x 7 x 10] + [5 x (6+1)] + 13

=[5 x 7 x 10] + [5 x 6] + [5 + 13]

=[5 x 7 x 10] + [5 x 6] + [18]

Each of the above term is an even number and all even numbers are composite numbers by definition. So sum of three composite numbers is also a composite number. hence proved.