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 −x22−x412.
(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.