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C1 differentiation

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    Morning everyone,

    I've got a question and my answer has revealed that I've got the wrong idea.

    'Determine the two pints on the curve with equation y=(x^2+3)(x-5) where the gradient is equal to -5.

    So this is what I did. I simplified the equation, differentiated it, then carried -5 over to the other side so I had a quadratic. I could factorise it so I put it through the quadratic equation. This must have been the wrong thing to do though as the answer is wrong
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    What answers did you get
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    what was the correct answer
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    (Original post by TenOfThem)
    What answers did you get

    I got  \frac{5 \pm \sqrt{31}}{3} for x. The correct answers for x were 2 or 1&(1/3)
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    (Original post by gavinlee)
    Morning everyone,

    I've got a question and my answer has revealed that I've got the wrong idea.

    'Determine the two pints on the curve with equation y=(x^2+3)(x-5) where the gradient is equal to -5.

    So this is what I did. I simplified the equation, differentiated it, then carried -5 over to the other side so I had a quadratic. I could factorise it so I put it through the quadratic equation. This must have been the wrong thing to do though as the answer is wrong
    Is this right?

    y=(x^2+3)(x-5) = y=x^3-5x^2+3x-15
    \frac{dy}{dx}= 3x^2-10x+3
    when \frac{dy}{dx} = -5 then 3x^2-10x+8=0
    which equals (3x-4)(x-2) so x=\frac{4}{3} or 2
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    What was your quadratic ... the book answers are correct


    Then again ... do not worry ... Rachel has taken away your chance to learn by posting the full answer
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    (Original post by rachel0593)
    Is this right?
    There is a good reason that the forum frowns on full solutions

    Gavin needed to learn where he was going wrong on this question ... he needed to unpick his misconceptions
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    (Original post by TenOfThem)
    There is a good reason that the forum frowns on full solutions

    Gavin needed to learn where he was going wrong on this question ... he needed to unpick his misconceptions
    Since when does that work?, learn all the marks schemes and apply your knowledge is the best way to get the top marks.
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    (Original post by Alex Dom)
    Since when does that work?, learn all the marks schemes and apply your knowledge is the best way to get the top marks.
    My mistake

    Clearly looking at other people's answers/the mark scheme is the way to succeed in maths exams

    How stupid am I

    Where on earth did I get my foolish view that understanding the material and the question was the way forward
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    Yeah I tried it again and got the right answers
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    (Original post by gavinlee)
    Yeah I tried it again and got the right answers
    ok
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    Sorry I though that by showing how I got to the answer he would see where he went wrong and could apply it to another question.
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    (Original post by TenOfThem)
    My mistake

    Clearly looking at other people's answers/the mark scheme is the way to succeed in maths exams

    How stupid am I

    Where on earth did I get my foolish view that understanding the material and the question was the way forward
    Just talking out of experience. He might learn that way too.
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    (Original post by rachel0593)
    Sorry I though that by showing how I got to the answer he would see where he went wrong and could apply it to another question.
    In that case ... sorry I snarled at you
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    (Original post by Alex Dom)
    Just talking out of experience. He might learn that way too.
    Durrrrrrr

    Again ... my stupidity ... of course with your vast experience you know what the best way for people to learn is
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    (Original post by gavinlee)
    I got  \frac{5 \pm \sqrt{31}}{3} for x. The correct answers for x were 2 or 1&(1/3)
    no no no i think you havent put the correct sign for -5
    thats the only way you get your answer
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    (Original post by Alex Dom)
    Since when does that work?, learn all the marks schemes and apply your knowledge is the best way to get the top marks.
    What are top marks in your opinion?

    I hardly do 1-2 past paper, i just have a quick view of the mark scheme of those papers, so does this means that i can never get top marks?


    (Original post by Alex Dom)
    Just talking out of experience. He might learn that way too.
    You look to be very experienced, can you give more advice on preparing maths?

    If someone follows your advice for A-Level maths, and then if he has to give STEP/AEA, than what shall he do to prepare?
    Learn all the STEP/AEA mark schemes?
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    (Original post by TenOfThem)
    Durrrrrrr

    Again ... my stupidity ... of course with your vast experience you know what the best way for people to learn is
    :lol:

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Updated: May 12, 2012
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