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Math Help - Negative/positive Roots?

  1. #1
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    Negative/positive Roots?

    I have these questions for an assignment we're meant to do using Microsoft Excel.


    Set up a table to find the negative root of x2 - 4x - 8 = 0

    Find another re-arrangement of the equation x2 - 4x - 8 = 0 to find the positive root by iteration.


    I have no idea what negative/positive roots are (the teacher never taught it) , and no clue how to finish this. If anyone could help I'd be extremely grateful.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Ingu View Post
    I have these questions for an assignment we're meant to do using Microsoft Excel.


    Set up a table to find the negative root of x2 - 4x - 8 = 0

    Find another re-arrangement of the equation x2 - 4x - 8 = 0 to find the positive root by iteration.


    I have no idea what negative/positive roots are (the teacher never taught it) , and no clue how to finish this. If anyone could help I'd be extremely grateful.
    A root of an equation is a value of the argument that satisfies it, a positive root is a root that is positive, and a negative root is one that is negative.

    Presumably you have been taught wat a root is, and you are expected to know what positive and negative mean.

    CB
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  3. #3
    Newbie chili5's Avatar
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    Your equation:

    x2 - 4x - 8 = 0

    means that there are 2 values of "x" that make the equation true. So when you solve the equations you would get 0 = 0.

    The idea of a root is if you graph the function you will find a parabola that crosses in 2 places. These are the roots. However you may find that there is only one root, or no roots.

    You can find the two roots by factoring the equation, or by using the quadratic formula!

    Quadratic equation - Wikipedia, the free encyclopedia
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  4. #4
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    Quote Originally Posted by Ingu View Post
    I have these questions for an assignment we're meant to do using Microsoft Excel.


    Set up a table to find the negative root of x2 - 4x - 8 = 0

    Find another re-arrangement of the equation x2 - 4x - 8 = 0 to find the positive root by iteration.


    I have no idea what negative/positive roots are (the teacher never taught it)
    Your teacher probably didn't think he/she needed to teach you what positive and negative numbers are and that is all this means! a "negative root" is just a negative number that satisfies the equation.

    , and no clue how to finish this. If anyone could help I'd be extremely grateful.
    There are a number of ways to solve quadratic equations but your reference to a "table" and "excel" makes me think you are expected to just set up a table to calculate x^2- 4x- 8 for a number of values of x- and see which are close to 0.

    Since your next problem requires an "iteration", I assume that you have learned something about that- but there are a number of iterative methods of solving equations and I don't know which is meant here.

    I think the simplest would be to rewrite x^2- 4x- 8 as x^2- 4x= x(x-4)= 8 and so x= \frac{8}{x-4}. Choose some starting value for x, put it into the right hand side and calculate the "x" on the right side. Iterate with that new value of x, cross your fingers and hope that it converges!
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