Quantcast

Got Homework?

Connect with other students for help. It's a free community.

  • across
    MIT Grad Student
    Online now
  • laura*
    Helped 1,000 students
    Online now
  • Hero
    College Math Guru
    Online now

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

gjhfdfg

Find a rational zero of the polynomial function and use it to find all the zeros of the function. f(x) = x^4 + 3x^3 - 5x^2 - 9x - 2

  • one year ago
  • one year ago

  • This Question is Closed
  1. campbell_st
    Best Response
    You've already chosen the best response.
    Medals 1

    try f(-1) ...so if f(-1) = 0 then x = -1 is a rational zero... this uses the rational zero theory..... p/q p = factors of the constant q = factors of the leading term

    • one year ago
  2. campbell_st
    Best Response
    You've already chosen the best response.
    Medals 1

    to find all the zeros... use synthetic division or polynomial division |dw:1360356521817:dw|

    • one year ago
  3. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    @gjhfdfg Do you know the rational root theorem? If you have some polynomial \[P(x) = a_nx^n+a_{n-1}x^n-1+\dots+a_1x^1+a_0\] and all the coefficients are integers, and \(p/q\) is a rational zero (and \(p/q\) is reduced), then \(p\) is a factor of \(a_0\) and \(q\) is a positive factor of \(a_0\).

    • one year ago
  4. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    I have done the rational root theorem but I never got it.

    • one year ago
  5. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Also, if \(a_n =1\) then all rational zeros are integers which divide \(a_0\).

    • one year ago
  6. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    So here \(a_0 = -2\) and that leaves only 4 roots to try: \(-1/1, -2/1, 1/1, 2/1\) Check the 1,-1 cases first, to make the arithmetic easier.

    • one year ago
  7. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Im lost,

    • one year ago
  8. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    I did the synthetic division but Im not sure what to do after, |dw:1360357822967:dw|

    • one year ago
  9. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    I don't remember synthetic division, I always do it the long division way :-) \[x^4 + 3x^3 - 5x^2 -9x -2\]We can take out x^3(x+1)\[2x^3-5x^2-9x-2\]We can take out 2x^2(x+1)\[-7x^2-9x-2\]We can take out -7x(x+1)\[-2x-2\]We can take out -2(x+1) so our quotient is \[x^3+2x^2-7x-2\]Now you can repeat the whole process of picking a root, factoring it out, wash, rinse, repeat...

    • one year ago
  10. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Long division, oh boy haha.

    • one year ago
  11. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Ok, so do I continue to take out x+1 from the rest?

    • one year ago
  12. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    You could try, but do you think x=-1 is a root of \[x^3 + 2x^2 - 7x - 2 = 0\]If it isn't, you won't get far trying to divide out (x+1)...

    • one year ago
  13. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    well, it won't come evenly, at least.

    • one year ago
  14. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Use the RRT to pick your next root to factor out.

    • one year ago
  15. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Because you've got the same leading coefficient, and the same trailing coefficient, you've got the same list of tentative roots to try: 1, -1, 2, -2

    • one year ago
  16. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    I dont really understand the rational root therom..

    • one year ago
  17. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Okay, here's an intuitive way to think about it: we know that if we have \[P(x) = (x-a)(x-b) = 0\]\[P(x) = x^2 -ax - bx + ab = x^2 -(a+b)x + ab\]Right? Similarly, if we had \[P(x) = (x-a)(x-b)(x-c) = 0\]\[P(x) = x^3 - (a+b+c)x^2 + (ab + ac + bc)x -abc\]and so on. You can see that if the coefficient of the leading term is 1, then the coefficient of the last term is just the product of the various zeros, right?

    • one year ago
  18. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Confusing

    • one year ago
  19. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    We may have more candidates than we do roots, of course, which is why we have to do some trial and error. For example, if -abc = 1, and we know all the roots are integers, then our job is very easy: there's a root that = 1, and one that = -1, and another one that equals 1. Does any of it make sense to you? Maybe I can build on that...

    • one year ago
  20. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Are you comfortable with the notion that a polynomial with roots a, b, c, etc. can be written \[P(x) = (x-a)(x-b)(x-c)\dots = 0\]?

    • one year ago
  21. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    No, the a,b,c really confuses me

    • one year ago
  22. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Well, okay, let's say we have a polynomial with roots x = 1, and x = 2. That means that at x = 1, the polynomial = 0, and at x = 2, the polynomial = 0. Good so far?

    • one year ago
  23. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Why do they equal 0?

    • one year ago
  24. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    that's the definition of a root of a polynomial P(x) is that at the root, P(root) = 0.

    • one year ago
  25. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    So for our polynomial with roots at x = 1, x = 2, P(1) = 0 and P(2) = 0.

    • one year ago
  26. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Okay, got it so far, I think..

    • one year ago
  27. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Good! Okay, there's a theorem that says if our polynomial has \(n\) roots, the highest order term in the polynomial will be \(x^n\) (and vice versa).

    • one year ago
  28. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    I don't know best how to explain some of this, but you can take it as a given that our polynomial with roots x = 1 and x =2 can be written\[P(x) = (x-1)(x-2) \]and it is easy to see that \(P(x) = 0\) if \(x = 1\) or \(x = 2\) right?

    • one year ago
  29. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Okay got it so far to here,

    • one year ago
  30. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    This is why if we can factor a polynomial into that form, we can find the roots easily by setting each of those product binomials \((x-1) = 0\), \((x-2) = 0\) and so on.

    • one year ago
  31. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Still with me?

    • one year ago
  32. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Im with you so far,

    • one year ago
  33. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Basically, if we have some number of expressions multiplied together, and the product is 0, we know that one or more of those expressions must also be 0, and we can find all the ways to make the whole thing be 0 by finding the solution for each expression to = 0.

    • one year ago
  34. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    So as long as we believe that we can write our polynomial in that form (this is the part I don't want to try to prove here), we're in good shape! So believe that, okay? :-)

    • one year ago
  35. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Now back to the RRT and how we could pick roots to try from some big ugly polynomial that is multiplied out! We've got our little "test" polynomial with roots x = 1 and x = 2. \[P(x) = (x-1)(x-2)\]Let's multiply that out and see what we get:\[P(x) = (x-1)(x-2) = x^2 - 2x - 1x + 2 = x^2 - 3x + 2\] Agreed?

    • one year ago
  36. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Agreed, I need to run to the store right quick, will you be still be here in about an hour?

    • one year ago
  37. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    Where did the value of 2 as the final coefficient come from? Well, it came exclusively from multiplying the roots! Let that sink in while you go to the store. I'll be back later on as well, but will continue writing even if you aren't here.

    • one year ago
  38. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    What if we decide to add x = -3 as a root to our polynomial? We can multiply the whole thing by \((x+3)\): \[P(x) = (x-1)(x-2)(x+3) = (x+3)(x^2-3x+2) = \]\[P(x) = x^3-3x^2+2x+3x^2-9x+6\]\[P(x)= x^3-7x+6\] Again, the trailing coefficient comes entirely from the multiplication of the roots. Let's throw in one more root, x = -5: \[P(x) = (x-1)(x-2)(x+3)(x+5) = (x^3-7x+6)(x+5)\]\[P(x) = x^4+5x^3-7x^2-35x+6x+30\]\[P(x) = x^4+5x^3-7x^2-29x+30\] Once again, that final term (30) came from multiplying our roots together. Let's say we found that \(P(x)\) on our homework assignment, and were told to find the roots? I don't know about you, but I can't just guess numbers that will work (well, okay, in this case I would try 1 as a guess, and it would work, but in general...) So we could use the RRT to get some numbers to try. What are the factors of 30? 1, 2, 3, 5, 6, 10, 15, 30 (and the negative numbers as well). We know that the zeros are on that list, because that final factor came just from multiplying the zeros together. Unfortunately, there are more values on the list than there are zeros :-)

    • one year ago
  39. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    But I hope you can see at this point that if we can get 1 root of the big polynomial, we can divide out (x - root) from the whole thing and get a polynomial of a lower degree, at which point we can repeat the process over and over until we have found all of them.

    • one year ago
  40. whpalmer4
    Best Response
    You've already chosen the best response.
    Medals 1

    And as we divide out (x-root) the number of possible factors of the final term gets smaller, at least if the root is some number other than 1 :-)

    • one year ago
  41. gjhfdfg
    Best Response
    You've already chosen the best response.
    Medals 0

    Makes a little more sense, still confused on finding the zeros? Thank you for taking your time out & explaining this to me.! :-)

    • one year ago
    • Attachments:

See more questions >>>

Your question is ready. Sign up for free to start getting answers.

spraguer (Moderator)
5 → View Detailed Profile

is replying to Can someone tell me what button the professor is hitting...

23

  • Teamwork 19 Teammate
  • Problem Solving 19 Hero
  • You have blocked this person.
  • ✔ You're a fan Checking fan status...

Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.

This is the testimonial you wrote.
You haven't written a testimonial for Owlfred.