Question

(a) Find the remainder when 15! is divided by 17. (b) Find the remainder when 2(26!) is divided by 29, Thatermine whether 17 is a prime by deciding whether 16! = -1

Answer 1

Explanation:

a) We have 15! as the product of 1 to 15 natural numbers. Since 17 is prime there will be no factor common to these

By actual division we find

15! (mod 17) =16

From this we deduce

even 16! mod 17 = 16 = -1

According to Wilson theorem

(17-1)! = -1 mod 17

Thus verified 17 is prime

Hence 15! (mod 17) =-1=16

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b) 2(26!) is divided by 29

Since 29 is prime

(29-1)! = -1 mod 29

28! = -1 mod 29 = 28

When divided this gives 25 as remainder