1. ## Differentiate

I don't understand the attached question. There might be a misprint.

2. $f(x) = 6 \sin (x) + \sqrt{2} \cos (x)$

$f'(x) = 6 \cos (x) - \sqrt{2} \sin (x)$

$f''(x) = -6 \sin (x) - \sqrt{2} \cos (x) = -[6 \sin (x) + \sqrt{2} \cos (x)] = -f(x)$

3. Thanks, I can see now its a standard notation for the second derivative.