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    This is the question I'm trying to do
    -----------------------------------------------------------
    in an experiment these results are obtained

    x = 1 2 3 4 5
    y= 9 23.1 60.9 135 257

    it is believed that a formula exists such as y = ax^3 + b connects y and x. Test to find if the results do satisfy such a formula, and find the values of a and b
    ----------------------------------------------

    So I cubed the x values and then did a plot on graph paper. This wasn't very practical so I did something that I'd learnt in S1 about regression lines. I used a calculator and used the y = a + bx facility. I got y = -3.5 +0.5x. I suppose that's 0.5 x^3.

    The answer in the book is a= -1/3 and b = 7/2

    What am I not understanding?

    Thanks
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    (Original post by maggiehodgson)
    ...
    Both answers are incorrect. Consider  y=az+b where  z=x^3 .
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    (Original post by maggiehodgson)
    This is the question I'm trying to do
    -----------------------------------------------------------
    in an experiment these results are obtained

    x = 1 2 3 4 5
    y= 9 23.1 60.9 135 257

    it is believed that a formula exists such as y = ax^3 + b connects y and x. Test to find if the results do satisfy such a formula, and find the values of a and b
    ----------------------------------------------

    So I cubed the x values and then did a plot on graph paper. This wasn't very practical so I did something that I'd learnt in S1 about regression lines. I used a calculator and used the y = a + bx facility. I got y = -3.5 +0.5x. I suppose that's 0.5 x^3.

    The answer in the book is a= -1/3 and b = 7/2

    What am I not understanding?

    Thanks
    The dat almost exactly fit y=2x^3+7
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    (Original post by maggiehodgson)
    I did something that I'd learnt in S1 about regression lines. I used a calculator and used the y = a + bx facility. I got y = -3.5 +0.5x. I suppose that's 0.5 x^3.
    This would have worked but you have entered the y values in the x column and x^3 values in the y column.

    Try it again and you will see that as brianeverit says y=2x^3+7 is a very good fit.
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    Thank you all.

    I made 2 errors. Firstly, I looked at the wrong set of answers in the book. Secondly, I did indeed enter the y values in the x column and the x^3 values in the y column.

    It all works now.
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    (Original post by maggiehodgson)
    Thank you all.

    I made 2 errors. Firstly, I looked at the wrong set of answers in the book. Secondly, I did indeed enter the y values in the x column and the x^3 values in the y column.

    It all works now.
    What are FP1 linear laws? I've looked over Fp1 vidos+ spec and have found nothing of the sort.
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    (Original post by MathMeister)
    What are FP1 linear laws? I've looked over Fp1 vidos+ spec and have found nothing of the sort.
    try looking in the AQA FP1 specification.
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    (Original post by MathMeister)
    What are FP1 linear laws? I've looked over Fp1 vidos+ spec and have found nothing of the sort.
    newton interpolation formula or lagrange

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    (Original post by MathMeister)
    What are FP1 linear laws? I've looked over Fp1 vidos+ spec and have found nothing of the sort.

    I'm doing AQA. Don't know if it's in your syllabus. I'm using a book called Advanced Maths for AQA Further Pure 1 and it's the last chapter. I've been looking at past papers and on the 4 I've looked at there is a question on it. For example June 07 question 5.

    Hope this helps. Perhaps you've done it but call it something else. I'm not in a school so I call it what the book calls it.
 
 
 
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