2013 THREE(c)

Merit
Question
Find the value of xx that gives the maximum value of the function

f(x)=50x30xln2xf(x)=50x-30x\ln 2x

You do not need to prove that your value of xx gives a maximum.

You must use calculus and clearly show your working, including any derivatives you need to
find when solving this problem.
Official Answer
(c)(c)

f(x)=50(30ln2x+30x1x)f'(x)=50-\left(30\ln 2x+30x\cdot \frac{1}{x}\right)

=2030ln2x\qquad=20-30\ln 2x

Maximum when f(x)=0f'(x)=0

20=30ln2x20=30\ln 2x

23=ln2x\frac{2}{3}=\ln 2x

x=e232=0.974x=\frac{e^{\frac{2}{3}}}{2}=0.974
Grading Criteria

Achievement (u)

  • Correct derivative.

Merit (r)

  • Correct solution with correct derivative.

Excellence T1

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Excellence T2

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