Saturday
September 20, 2014

Homework Help: math

Posted by mathstudent on Tuesday, January 2, 2007 at 1:59am.

Factor: x^3 - 3/4x - 1/4

The answer is: (x - 1)(x + 1/2)^2

How do I learn to do that? I'd like to reread an appropriate chapter from an appropriate textbook and do practice problems.


It takes experience and practiced eye. Algebra books have chapters on factoring; ask your teacher to borrow his/her desk copies. Also search google, such as library thinkquest for material.

You first multiply the equation by 4 so that all the coefficients are integers:

4x^3 - 3x - 1

You then reason as follws. Suppose that this function has a rational root. I.e. there exists integers p and q that are reletively prime (i.e. they don't have any prime divisors in common) such that p/q is a root. Then it can be shown (I'll give the proof below) that:

p must divide -1

q must divide 4

So all the candidates for rational roots are:

+/- 1, +/- 1/2 and +/-1/4

If yopu find one root, say x = 1, then you just divide 4x^3 - 3x - 1 by x - 1 using e.g. long division and you then find a quadratic equation that you can factor using elementary means and you'll obtain the factor (x + 1/2)^2


Proof:

Suppose you have an equation of the form

a_{n} x^n + a_{n-1} x^(n-1) + ...a_{1}x + a_{0} = 0

where all the a_{j} are integers. Suppose that a solution is x = p/q where p and q are integers that are relatively prime. Inserting x = p/q and multiplying by q^n gives:

a_{n} p^n + a_{n-1} p^(n-1)q + ...a_{1}pq^(n-1) + a_{0}q^n = 0

Let's write this as:

a_{n} p^n + a_{n-1} p^(n-1)q + ...a_{1}pq^(n-1) =- a_{0}q^n

Note that the left hand side is divisible by p:

[a_{n} p^(n-1) + a_{n-1} p^(n-2)q + ...a_{1}q^(n-1)]p = =- a_{0}q^n

The square brackets on the left hand side is an integer.

This then means that the right hand side must also be divisible by p. But q has no factors in common with p, so a_{0} must be divisible by p!

You can also easily see that a_{0} must be divisible by q by rewriting the equation as:

a_{n-1} p^(n-1)q + ...a_{1}pq^(n-1) + a_{0}q^n = -a_{n} p^n

You now observe that the left hand side is divisible by q, therefore the right hand side must also be divisible by q. Because p has no factors in common with q you can conclude that a_{n} must be divisible by q.

Correction:

"You can also easily see that a_{0} must be divisible by q by rewriting the equation as:"

I meant a_{n}, of course :)




that's very helpful. Thank you so much for the assistance!

Count Iblis, that was presented to the student in a most informative and helpful way. Thanks.

Thanks! It's a pleasure to help here!

You are welcome :)

Answer this Question

First Name:
School Subject:
Answer:

Related Questions

math - How do I figure out 6c=__?__pt showing work See my other response. =) ...
Edu 310 - When is it appropriate to learn individually and when is it ...
Children's Lit. - I asked this question today, and made a mistake in my question...
data management issues - Which of the following is not a source for external ...
English - What kind of movie do you like? Why? 1. I like comedy movies because I...
Chemistry - I Have Two Questions 1)How would you calculate the number of ...
English 2 - 1)In anthem what are the implications of his creation? (Chapter 5) 2...
math,correction - can someone check these for me please.... problem#4 Directions...
math - If the sides of a box all double in length, by how much does its volume ...
math,correction - Can someone correct these for me plz... Problem#6 Factor. x(x-...

Search
Members