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  • Perplex

    Paper 3 Practice

    [Maximum mark: 23]

    This questions asks you to investigate the recursive nature of the derivatives of ​1−x1​​ and how this impacts the Maclaurin series expansion.


    Consider the function ​f(x)=1−x1​, ​x=1.

    1. Find the value of ​f(0.5).

      [1]

      To earn a crown, get your answer ready before you reveal the options!

    2. Show that ​(1−x)f′(x)−f(x)=0.

      [2]
    3. By differentiating the equation in (b), show that ​(1−x)f′′(x)−2f′(x)=0.

      [3]

    Let ​f(n)​ be the ​nth​ derivative of ​f.

    1. Using mathematical induction, prove that ​(1−x)f(n)(x)−nf(n−1)(x)=0​ for all ​n∈N.

      [6]
    2. Hence solve the differential equation ​dxdy​=1−x3y​.

      [3]
    3. Using (d), show that ​f(n)(0)=n!​ for all ​n∈N.

      [2]
    4. Hence find the Maclaurin series expansion for ​f(x).

      [3]
    5. Use the Maclaurin series expansion to find the value of ​f(21​), and comment on the result.

      [3]

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