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Longhand

Chapter 3: Applications of derivatives

Linear approximation and Taylor polynomials

A differentiable function looks like its tangent line close to the point of tangency, looks more like a parabola built from its second derivative, and so on. Exams ask for the estimate, for the polynomial, and, the part most often lost, for a bound on how wrong the estimate can be.

Practice questions

Every question below is original material written for Longhand. Open a solution to read the full working, or answer it yourself to have it counted towards your progress.

  • Introductory1 mark · 2 min

    Use a linear approximation of f(x)=xf(x) = \sqrt{x} at x=25x = 25 to estimate 26\sqrt{26}.

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  • Introductory1 mark · 2 min

    The linear approximation of a function ff about x=2x = 2 is L(x)=4x7L(x) = 4x - 7. What are f(2)f(2) and f(2)f'(2)?

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  • Introductory2 marks · 3 min

    The third-order Maclaurin polynomial of a function ff is given below. Find f(0)f'''(0).

    T3(x)=2x+5x243x3T_{3}(x) = 2 - x + 5x^{2} - \tfrac{4}{3}x^{3}
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  • Standard2 marks · 3 min

    Use a linear approximation to estimate (3.002)4(3.002)^{4}.

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  • Standard2 marks · 4 min

    Use a linear approximation to estimate arctan(0.9)\arctan(0.9), using the fact that arctan1=π4\arctan 1 = \dfrac{\pi}{4}.

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  • Standard3 marks · 4 min

    A function ff has f(1)=3f(1) = 3, f(1)=2f'(1) = -2 and f(1)=6f''(1) = 6. Use the quadratic approximation of ff centred at x=1x = 1 to estimate f(1.1)f(1.1).

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  • Standard3 marks · 5 min

    Find the coefficient of x3x^{3} in the third-order Maclaurin polynomial of f(x)=(1+2x)1/2f(x) = (1 + 2x)^{-1/2}.

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  • Challenging3 marks · 6 min

    A function ff has third derivative f(x)=cosx4+x2f'''(x) = \dfrac{\cos x}{4 + x^{2}}. If f(0.5)f(0.5) is approximated by its second-order Maclaurin polynomial T2(0.5)T_{2}(0.5), find the best bound on the error f(0.5)T2(0.5)|f(0.5) - T_{2}(0.5)| that the Taylor remainder formula gives.

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  • Challenging4 marks · 7 min

    The equation y3+xy=10y^{3} + xy = 10 defines yy implicitly as a function of xx near the point (1,2)(1, 2). Use the tangent-line approximation at that point to estimate yy when x=1.1x = 1.1.

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  • Capstone5 marks · 10 min

    Use the second-order Taylor polynomial of f(x)=xf(x) = \sqrt{x} about x=16x = 16 to estimate 17\sqrt{17}, and use the Taylor remainder to give a bound on the error of your estimate.

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