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    hey, me again with a maths question :confused:

    basically i watched this video on factorising cubics:
    https://www.youtube.com/watch?v=RIHqIjt9jy0

    it essentially finds the first factor then uses long division.

    i understand that method totally, but when i tried it on this question:

    f(x) = 2x^3 + x^2 -8x - 4

    i found that f(2) was a factor (the equation came to 0). in the video it says to use the negative version of it (so x-2).

    however, when i used x-2 in long division it came out with the wrong answer, and in the answer book it uses x+2!

    how come in this circumstance they don't use the negative version?

    send help!!
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    (Original post by louisedrake)
    send help!!
    What you've done is entirely correct, it is a coincidence that in this case, both f(2) = f(-2) = 0 so that using x-2 or x+2 is correct.

    In fact, your polynomial factorises as f(x) = (x+2)(x-2)(2x+1), so the book using the negative version, except with x - (-2). But your method is just as correct.
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    (Original post by louisedrake)
    hey, me again with a maths question :confused:
    So, if you used x-2 then you should (after long division) get (x+2)(2x+1)

    If you used x -(-2) = x + 2 then you should (after long division) get (x-2)(2x+1).
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    (Original post by Zacken)
    What you've done is entirely correct, it is a coincidence that in this case, both f(2) = f(-2) = 0 so that using x-2 or x+2 is correct.

    In fact, your polynomial factorises as f(x) = (x+2)(x-2)(2x+1), so the book using the negative version, except with x - (-2). But your method is just as correct.
    thanks so much! x
 
 
 
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