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Ok, I'm trying to teach my kid how to factor polynomials. I already know how to factor bi- and tri- nomials and I already know how to take the GCF (Greatest Common Factor), for instance:

With the equation:

x^3 - 7x^2 - 16x + 112

This can be grouped into (x^3 - 7x^2) - (16x -112)

Then can factor out:

x^2(x - 7) - 16(x - 7)

Which then becomes:

(x^2 - 16)(x - 7)

And finally:

(x - 4)(x + 4)(x - 7)

That's all well and good... but what about something like:

x^3 + 3x^2 - 46x - 168

In this case, if I KNOW any of the factors already, I can determine the rest... but how do I get a factor to begin with? The GFC doesn't seem to work for this...

And, again, I want to point out this is me trying to learn how to teach someone else, not actual homework...

(And to demonstrate that I know the answer):

(x+ 6)(x - 7)(x + 4)

With the equation:

x^3 - 7x^2 - 16x + 112

This can be grouped into (x^3 - 7x^2) - (16x -112)

Then can factor out:

x^2(x - 7) - 16(x - 7)

Which then becomes:

(x^2 - 16)(x - 7)

And finally:

(x - 4)(x + 4)(x - 7)

That's all well and good... but what about something like:

x^3 + 3x^2 - 46x - 168

In this case, if I KNOW any of the factors already, I can determine the rest... but how do I get a factor to begin with? The GFC doesn't seem to work for this...

And, again, I want to point out this is me trying to learn how to teach someone else, not actual homework...

(And to demonstrate that I know the answer):

(x+ 6)(x - 7)(x + 4)

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I guess when I said "trying to teach my kid" I should have mentioned he's 14...

I don't even understand the numerical method (what level of math is that?)...

As for the analytical method, do you mean that that is the EASY way of doing it? Meaning there isn't a method as simple as there is for the bi- or tri-...?

I'm still working through that method to see if I understand it...

(Boy, I feel stupid...) =]

http://www.wtamu.edu/academic/anns/mps/math/mathlab/col_algebra/col_alg_tut7_factor.htm

Nothing too complicated for a 14 year old (I hope).

>> x^3 + 3x^2 - 46x - 168

For your specific example, none of the standard methods turn up something, so the easiest is to revert to guessing (since we know that the zero's will be whole numbers, this shouldn't take too long). The first one I found was -4 (I started with negative values because it seemed most likely to cancel out the large negative -168 term, and the first one I tried was -4, since 4 * 46 > 168 - it turned out that my guess was good ;) ) :

(x + 4) (x² - x - 42)

The trinomial can be factored by "guessing again" (the -6 and 7 zero's can be easily "guessed"), or by using the quadratic formula :

http://en.wikipedia.org/wiki/Quadratic_equation

I actually already saw that page while searching for a method...

I knew about (and explained to) the method where you trial and error using factors, ie:

6x^2 + 23x + 20

6 can be 1 & 6, 2 & 3 or vice versa or their negatives...

20 can be 1 & 20, 2 & 10, 4 & 5 or vice versa or their negatives...

Trial and error each different combination if needed (or as skill improves, automatically ignore some)...

So, no guaranteed method like that for finding the answer in this case, right...?

The reason the Q came up for me is I saw some problems coming up for him that provide one or two factors and ask you to determine the rest - I was expecting there might be a next step where none was provided and some relatively easy (and new) method might have to be learned to figure all of them...

Well, there are the links ozo posted. But they're a bit more advanced and more complicated (too complicated for this kind of simple factoring exercise imo).

>> I was expecting there might be a next step where none was provided

That seems likely, yes. Unfortunately, the "guess" method works best in exercises like these, especially if you practice a lot (and thus gain experience, and can pretty much "see" the correct solution).

http://www.sosmath.com/algebra/factor/fac10/fac10.html

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Start your 7-day free trialDividing the polynomial by the provided factors would give you a simpler polynomial to factor the rest

>> x^3 + 3x^2 - 46x - 168 = f

for X small enough, (say x=-10) f is surely negative (-1000 + 300 + 460 ...)

for X big enough, (for ex. x=10) f is surely positive (1000+300-460...)

teach your boy to draw 2 dots on a 2-D grid at (-10,f(-10)) and (+10,f(10))

tell him that a line must connect the 2 dots (fermat...) so the 1st root must be somewhere between the values.

another trick : set X=0 and find f(0) = - 168 = 2*2*2*3*7

168 = 1 * 2 * 84 = 1 * 3 * 56 = 1 * 4 * 42 = 1 * 6 * 28 = 1 * 7 * 24 = 1 * 8 * 21 = 1 * 12 * 14 =

= 2 * 2 * 42 = 2 * 3 * 28 = 2 * 4 * 21 = 2 * 6 * 14 = 2 * 7 * 12 =

= 3 * 4 * 14 = 3 * 7 * 8 =

= 4 * 6 * 7

now try to look for 3 numbers and with + and - signs to reach 3 (coef' of X^2)

easily you can remove the triplets with large numbers like 84,56,42,28,24,21,

but some triplets can do it: 3=14-12+1 = 12- 7 -2 = 4 - 7 + 6

X=+12 or -12 is too large/small (see the graph your kid already draw)

we remain with 4,6,7 : check it.

if u realy wants to teach your son, empower his arithmetical skills first, and later get him used to draw his problems (were it can be draw)

luck!

An alternative to looking for all 3 numbers at once is to look for the numbers one at a time.

If any one of the numbers is a root, you can remove it and be left with a simpler polynomial to find the remaining roots.

(And once you reduce it to a quadratic, the quadratic formula is much simpler than the cubic formula)

you can always use this method

http://en.wikipedia.org/wiki/Bisection_method

If you only want integer roots, you can stop when your interval size becomes less than 1

thx...

Math / Science

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http://mathworld.wolfram.com/NewtonsMethod.html

analytically

http://www.dpmms.cam.ac.uk/~wtg10/cubic.html