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Date May 2008 Marks available 14 Reference code 08M.3ca.hl.TZ2.4
Level HL only Paper Paper 3 Calculus Time zone TZ2
Command term Find and Show that Question number 4 Adapted from N/A

Question

(a)     Given that y=lncosx , show that the first two non-zero terms of the Maclaurin series for y are x22x412.

(b)     Use this series to find an approximation in terms of π for ln2 .

Markscheme

(a)     f(x)=lncosx

f(x)=sinxcosx=tanx     M1A1

f     M1

f'''(x) = - 2\sec x\sec x\tan x     A1

{f^{iv}}(x) = - 2{\sec ^2}x({\sec ^2}x) - 2\tan x(2{\sec ^2}x\tan x)

= - 2{\sec ^4}x - 4{\sec ^2}x{\tan ^2}x     A1

f(x) = f(0) + xf'(0) + \frac{{{x^2}}}{{2!}}f''(0) + \frac{{{x^3}}}{{3!}}f'''(0) + \frac{{{x^4}}}{{4!}}{f^{iv}}(0) + …

f(0) = 0,     M1

f'(0) = 0,

f''(0) = - 1,

f'''(0) = 0,

{f^{iv}}(0) = - 2,     A1

Notes: Award the A1 if all the substitutions are correct.

Allow FT from their derivatives.

 

\ln (\cos x) \approx  - \frac{{{x^2}}}{{2!}} - \frac{{2{x^4}}}{{4!}}     A1

= - \frac{{{x^2}}}{2} - \frac{{{x^4}}}{{12}}     AG

[8 marks]

 

(b)     Some consideration of the manipulation of ln 2     (M1)

Attempt to find an angle     (M1)

EITHER

Taking x = \frac{\pi }{3}     A1

\ln \frac{1}{2} \approx - \frac{{{{\left( {\frac{\pi }{3}} \right)}^2}}}{{2!}} - \frac{{2{{\left( {\frac{\pi }{3}} \right)}^4}}}{{4!}}     A1

- \ln 2 \approx - \frac{{\frac{{{\pi ^2}}}{9}}}{{2!}} - \frac{{2\frac{{{\pi ^4}}}{{81}}}}{{4!}}     A1

\ln 2 \approx \frac{{{\pi ^2}}}{{18}} + \frac{{{\pi ^4}}}{{972}} = \frac{{{\pi ^2}}}{9}\left( {\frac{1}{2} + \frac{{{\pi ^2}}}{{108}}} \right)     A1

OR

Taking x = \frac{\pi }{4}     A1

\ln \frac{1}{{\sqrt 2 }} \approx - \frac{{{{\left( {\frac{\pi }{4}} \right)}^2}}}{{2!}} - \frac{{2{{\left( {\frac{\pi }{4}} \right)}^4}}}{{4!}}     A1

- \frac{1}{2}\ln 2 \approx - \frac{{\frac{{{\pi ^2}}}{{16}}}}{{2!}} - \frac{{2\frac{{{\pi ^4}}}{{256}}}}{{4!}}     A1

\ln 2 \approx \frac{{{\pi ^2}}}{{16}} + \frac{{{\pi ^4}}}{{1536}} = \frac{{{\pi ^2}}}{8}\left( {\frac{1}{2} + \frac{{{\pi ^2}}}{{192}}} \right)     A1

[6 marks]

Total [14 marks]

Examiners report

Some candidates had difficulty organizing the derivatives but most were successful in getting the series. Using the series to find the approximation for \ln 2 in terms of \pi was another story and it was rare to see a good solution.

Syllabus sections

Topic 9 - Option: Calculus » 9.6 » Maclaurin series for {{\text{e}}^x} , \\sin x , \cos x , \ln (1 + x) , {(1 + x)^p} , P \in \mathbb{Q} .

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