Prove True/False that n^3-n is Always Divisible By 6

In summary, the statement n3-n is always divisible by 6 for all natural numbers n is true and can be proven through a direct proof by factorising n3-n into three integer factors and using the divisibility rule for 3. The factors n-1, n, and n+1 are all consecutive integers.
  • #1
CheesyPeeps
36
2

Homework Statement


For all natural numbers n, prove whether the following is true or false:
n3-n is always divisible by 6.

From SQA Advanced Higher Mathematics 2006 Exam Paper

Homework Equations


I can choose from the following types of proof:
Direct proof
Proof by contradiction
Proof by contrapositive
Proof by induction

The Attempt at a Solution


I know the statement is true, but proving it has been more difficult than I thought it would be!
I tried proof by induction, but got stuck with trying to prove true for n=k+1. I then tried proving the statement true for n=2k (even number) and n=2m+1 (odd number), but again, I didn't seem to be getting anywhere.
Am I along the right lines, or should I be trying something different?
 
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  • #2
Try to factor ##n^3-n## and see what you can say about the factors.
 
  • #3
CheesyPeeps said:
Am I along the right lines
A simple direct proof is possible. Start by factorising ##n^3-n## into three integer factors.

EDIT: Now I am jinxed and will have to wait for somebody to say my name before I can speak aloud again.
 
  • #4
fresh_42 said:
Try to factor ##n^3-n## and see what you can say about the factors.

Okay, so I've factorised it and found that ##n^3-n## is always even and therefore always divisible by 2. In order for it to be divisible by 6, it must be divisible by 2 and 3, but I'm not sure how to go about proving that it's divisible by 3.

EDIT: I looked up the divisibility rule for 3, and found it expressed as ##n(n+1)(n-1)## which is exactly what I have from factorising ##n^3-n##!
 
Last edited:
  • #5
What kind of numbers are ##n-1\, , \,n\, , \,n+1##?
 

1. What is the equation for "Prove True/False that n^3-n is Always Divisible By 6"?

The equation is n^3-n.

2. What does it mean for a number to be divisible by 6?

A number is divisible by 6 if it can be evenly divided by 6 without leaving a remainder.

3. How do you prove that n^3-n is always divisible by 6?

To prove that n^3-n is always divisible by 6, you can use mathematical induction. First, show that the equation is true for n=1. Then, assume it is true for some arbitrary value of n=k. Finally, prove that it is also true for n=k+1. This will show that the equation is true for all positive integers and therefore always divisible by 6.

4. Can you provide an example to illustrate that n^3-n is always divisible by 6?

For example, let n=3. Then, n^3-n=3^3-3=27-3=24. 24 is divisible by 6, therefore the equation holds true for n=3. This can be repeated for any positive integer value of n, showing that the equation is always divisible by 6.

5. Are there any exceptions to the rule that n^3-n is always divisible by 6?

No, there are no exceptions to this rule. The equation n^3-n will always result in a number that is divisible by 6 for any positive integer value of n.

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