1. ## derivative

f(x) = sinx/x^3

i obtained

f'(x) = (x²cos-3xsinx)/x^4

is that correct :O?

2. Using the quotient rule,

$\frac{d}{dx} \frac{\sin x}{x^3} =$

$\frac{x^3 \cos x - 3x^2 \sin x}{x^6} =$

$\frac{x \cos x - 3 \sin x}{x^4}$

which is not quite what you had, but I think you had the right idea.