2018 ONE(b)

Achievement
Question
If f(x)=3cos3xf(x)=3\cos 3x, show that 9f(x)+f(x)=09f(x)+f''(x)=0.
Official Answer
f(x)=9sin3xf'(x)=-9\sin 3x

f(x)=27cos3xf''(x)=-27\cos 3x

9f(x)+f(x)9f(x)+f''(x)

=9(3cos3x)27cos3x=9(3\cos 3x)-27\cos 3x

=27cos3x27cos3x=27\cos 3x-27\cos 3x

=0=0
Grading Criteria

Achievement (u)

  • Correct proof with correct first and second derivatives.

Merit (r)

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Excellence T1

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Excellence T2

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Video Explanation
NCEA Level 3 Calculus Differentiation 2018 NZQA Exam - Worked Answers by infinityplusone(starts at 1:56)
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