Find the derivative of the function \(f(x) = \sqrt{3x^2 + 2x + 1}\).
If $y = 5 \cos x - 3 \sin x$, prove that $\frac{d^2y}{dx^2} + y = 0$.
Show that \( f(x) = \tan^{-1}(\sin x + \cos x) \) is an increasing function in \( \left[ 0, \frac{\pi}{4} \right] \).