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    Q) Factorise 9 - 8x - x^2

    My working:
    x^2 + 8x - 9 = 0
    (x+9) (x-1)

    Markscheme asnwer:
    (9+x)(1-x)

    Does it matter that I worked it out as (x-1)? Is it the same thing? How would I get (1-x)?
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    you have factorized a different quadratic expression; they wanted you to factorize 9 - 8x - x2

    .... you can write it as

    -{ x2 + 8x - 9 } first of all...
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    (Original post by G.Y)
    Q) Factorise 9 - 8x - x^2

    My working:
    x^2 + 8x - 9 = 0
    (x+9) (x-1)

    Markscheme asnwer:
    (9+x)(1-x)

    Does it matter that I worked it out as (x-1)? Is it the same thing? How would I get (1-x)?
    There is no equation in the question, however you wrote = 0 in your answer. This isn't good notation.

    of course it matters, substitute some arbritrary value of x and you'll see that the expressions are not the same.

    Now, you can't just multiply by -1 because you'll be factorising a different expression. -(x-1) = 1-x, can you see the mistake?

    I'd start by writing  9 - 8x - x^2 = -(x^2 +8x -9)
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    (Original post by NotNotBatman)
    There is no equation in the question, however you wrote = 0 in your answer. This isn't good notation.

    of course it matters, substitute some arbritrary value of x and you'll see that the expressions are not the same.

    Now, you can't just multiply by -1 because you'll be factorising a different expression. -(x-1) = 1-x, can you see the mistake?

    I'd start by writing  9 - 8x - x^2 = -(x^2 +8x -9)
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    I couldn't get it
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    (Original post by G.Y)
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    I couldn't get it
    Correct. The answer is just written in the same form, but they are equivalent.

    In the solution the other bracket was multiplied by-1, but they are equivalent.
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    (Original post by NotNotBatman)
    Correct. The answer is just written in the same form, but they are equivalent.

    In the solution the other bracket was multiplied by-1, but they are equivalent.
    (-x-9) is the same as (x+9) ?? Why
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    (Original post by G.Y)
    (-x-9) is the same as (x+9) ?? Why
    It's not.

    Your last is incorrect, you distributed the negative into BOTH brackets, but it should only be distributed into one or the other, because this is a product of the two terms, not a sum.

    You're correct up to -[(x+9)(x-1)]. Now distribute the minus at the front into the second bracket, granting (x+9)(1-x) as required.
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    (Original post by G.Y)
    (-x-9) is the same as (x+9) ?? Why
    Oh, Ignore my previous post.

    On the line where you multiplied the bracket by -1, you should only multiply one of the brackets by -1, you can choose which one.

    Otherwise you'll be multiplying by (-1)^2 =1
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    (Original post by RDKGames)
    It's not.

    Your last is incorrect, you distributed the negative into BOTH brackets, but it should only be distributed into one or the other, because this is a product of the two terms, not a sum.

    You're correct up to -[(x+9)(x-1)]. Now distribute the minus at the front into the second bracket, granting (x+9)(1-x) as required.
    Does it make a difference which bracket? And how would you know which one if it does?
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    (Original post by G.Y)
    Does it make a difference which bracket? And how would you know which one if it does?
    It does not make a difference, but the mark scheme way have it distributed in a different one. You wouldn't lose any marks if this is the case.
 
 
 
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