I don't understand the attached question. There might be a misprint.
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$\displaystyle f(x) = 6 \sin (x) + \sqrt{2} \cos (x)$ $\displaystyle f'(x) = 6 \cos (x) - \sqrt{2} \sin (x)$ $\displaystyle f''(x) = -6 \sin (x) - \sqrt{2} \cos (x) = -[6 \sin (x) + \sqrt{2} \cos (x)] = -f(x)$
Thanks, I can see now its a standard notation for the second derivative.
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