Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

Pages: **1**

**dean****Guest**

I need to prove that if p is a prime number ending on 1 (like 11, 101), p+4 is a multiple of 15. It is obvious that it's a multiple of 5, so i just have to prove that it's also a multiple of 3. But how? I have really tried, but I just can't work it out.

dean

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,660

Haven't worked it out for you yet, but you could try looking for a proof that p+1 is divisible by 3 (because if p+4 is, then p+1 is and vice versa)

Another thing: Prime Numbers are either 6n-1 or 6n+1 (n being an integer), because 6n is divisible by 6, 6n+3 is divisible by 3 and 6n+2 and 6n+4 are divisible by 2, leaving only 6n+1 or 6n+5 (which is a type of 6n-1)

So there is a fair chance that a prime plus one will be divisible by 6 (and hence 3) but not *proven*.

Umm ... let me see ... if the prime must end in 1 then the case p-1 is divisible by 10, right? That gets us close ...

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

Offline

**santhoshkumar****Guest**

respected sir im a guy form india. have question about the prime

number

,here is my question

when does the prime number comes after what type

of number does the prime

number come.if found in a web site that

primenumber expect 2,3 comes before

or after a number divisible by 6.

but some time it is right a many times its

wrong. and i have another

question when does the even number and odd number

come after what type

of number can we get the odd and even what are therules to find out the

next number.kindly do this as my kind request .i hope

u will help me.

this would me very usefull if u do this .

your's faith full'

r.santhoshkumar

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,660

Such a nice request, but noone answered it? It slipped under my radar.

Ummm ...

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

Offline

**Roraborealis****Member**- Registered: 2005-03-17
- Posts: 1,594

School is practice for the future. Practice makes perfect. But - nobody's perfect, so why practice?

Offline

Pages: **1**