Factoring something with cube power?

In summary, the conversation discusses finding the intersection of two equations, x = y^3 and x = 3y + 2. The rational root theorem is mentioned as a way to find possible factors, and the conversation also explores the use of substitution and long division to find factors. Ultimately, it is determined that y+2 is a factor of the polynomial y^3 - 3y + 2.
  • #1
zeion
466
1

Homework Statement



I need to find where these two intersect:
x = y^3
x = 3y + 2

Homework Equations





The Attempt at a Solution




I set them to equate:
y^3 = 3y + 2
y^3 - 3y + 2 = 0

How do I factor this??
 
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  • #2
I just realized I can sub in 2 to get a factor..
But is there a faster way than to guess randomly?
 
  • #3
you can't factor that. But what you could do is try to guess the simplest number you can. and if that number were 1, you would be right and see that it is a factor.

For no reason whatsoever you could also try -2, and you would see that it is also a factor

but that's just a quick evaluation, if you wanted the third factor you could use division of polynomials and hopefully you would get a 2nd degree polynomial which is easily factored
 
  • #4
zeion said:
I just realized I can sub in 2 to get a factor..
But is there a faster way than to guess randomly?

2 is not a factor
 
  • #5
dacruick said:
2 is not a factor

I suppose he meant: if you substitute y=2 in the polynomial you get 0, so (y-2) has to be a factor.

You can use the rational root theorem to find all rational numbers that might be a factor, in this case you only have to try -2,-1,1 and 2
 
  • #6
Ah sorry I made a mistake in the equation.. it's supposed to be
y^3 = 3y + 2
y^3 - 3y - 2 = 0
 
  • #7
What is rational root theorem?
How did you know those numbers to try?
I can always just do a long division right?
 
  • #8
There's a theorem about rational roots of polynomials. For a polynomial anxn + ... + a1x + a0 = 0, if x = p/q is a zero, then p must divide a0 and q must divide an.

With your polynomial, y3 - 3y + 2 = 0, where p/q is a rational root, p has to divide 2 and q has to divide 1. The possible choices for p are +/-1 and +/-2. This means that the possible rational zeroes are +/- 1 and +/- 2. No other rational zeroes are possible. I found that 1 is a root, meaning that y - 1 is a factor.
 
  • #9
So if I have a missing term in a polynomial I just have to try to factor with the +/- of the first and last coefficients?
 
  • #10
willem2 said:
I suppose he meant: if you substitute y=2 in the polynomial you get 0, so (y-2) has to be a factor.

You can use the rational root theorem to find all rational numbers that might be a factor, in this case you only have to try -2,-1,1 and 2

y-2 is not a factor still. y+2 is a factor however.
 

FAQ: Factoring something with cube power?

How do I factor something with a cube power?

Factoring something with a cube power involves finding the common factors of the expression and grouping them together. This is similar to factoring with a square power, but with an extra step of factoring out the cube root.

What is the difference between factoring with a square power and factoring with a cube power?

The main difference is that when factoring with a cube power, you need to factor out the cube root in addition to finding the common factors. This means that the final factors will include both the square root and the cube root of the original expression.

Can you provide an example of factoring something with a cube power?

Sure, let's take the expression 27x^3 + 9x^2. The first step is to find the common factor, which in this case is 9. This leaves us with 9(3x^3 + x^2). Next, we factor out the cube root of the remaining terms, which is x^2. This gives us 9x^2(x + 3). Therefore, the final factored expression is 9x^2(x + 3).

What if the expression has a coefficient in front of the variable with the cube power?

If there is a coefficient in front of the variable with the cube power, you will need to factor it out first before finding the common factors. For example, if we have 8x^3 + 4x^2, we can factor out 4x^2 to get 4x^2(2x + 1). Then, we can proceed with factoring out the cube root and find the common factor as mentioned before.

Can all expressions with cube powers be factored?

No, not all expressions with cube powers can be factored. Some expressions may already be fully factored, while others may not have any common factors that can be factored out. In these cases, the expression cannot be factored any further.

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