# A,b,c satisfying given conditions

1. Apr 22, 2014

### utkarshakash

1. The problem statement, all variables and given/known data
Let a,b,c be distinct real numbers satisfying $a^3+b^3+6abc = 8c^3$ then which of the following may be correct?

A) a,c,b are in Arithmetic Progression
B) a,c,b are in Harmonic Progression
C) a+bω-2cω^2 =0
D) a+bω^2-2cω=0

3. The attempt at a solution

I could prove the first statement but I really can't figure out how to deal with the other options. Since the first statement is correct all I know is that 2c=a+b. Suppose I replace a with 2c-b in the third statement and simplify it, I get 4c-b+ω(2c+b) = 0. But I guess LHS can't be zero(I'm not sure about this).

2. Apr 22, 2014

### Saitama

The following should help:
$$x^3+y^3+z^3-3xyz=(x+y+z)(x+\omega y+\omega^2 z)(x+\omega^2 y+\omega z)$$
Can you figure out what $x,y,z$ are for the given problem? :)

3. Apr 22, 2014

### AlephZero

For B), do you remember an inequality between the arithmetic and geometric means of a set of numbers?

4. Apr 22, 2014

### Saitama

I am not sure but OP already proved that a,b,c are in AP. Why check the B case now? :/

5. Apr 23, 2014

### AlephZero

The question asks if a b and c may be in AP, GP, etc.

The OP proved they may be in AP. There could be other values that are in GP.

In fact they could be in a GP, except that the question says they are distinct real numbers.

NOTE: I'm can't remember what I was thinking about when I posted #3 - ignore it!

You can check for a GP by substituting $b = ka$, and $c = k^2a$.

6. Apr 23, 2014

### Saitama

It does state that they are distinct. :)

7. Apr 23, 2014

### AlephZero

You replied to my post #5 before I had finished editing the typos

8. Apr 23, 2014

### Saitama

So no need to check the other cases now. :)