$\text{f(x)}=$ $\frac{5}{x^3}+5\cdot \cos \left(x\right)$

= $5x^{-3}+5\cdot \cos \left(x\right)$

=> f'(x) = $-15x^{-4}-5\cdot \sin \left(x\right)$

$\text{f'(x)}=$ $-\frac{15}{x^4}-5\cdot \sin \left(x\right)$

$\text{f(x)}=$ $8\sqrt{x}$

= $8x^{\frac{1}{2}}$

=> f'(x) = $4x^{-\frac{1}{2}}$

$\text{f'(x)}=$ $\frac{4}{\sqrt{x}}$

$\text{f(x)}=$ $\frac{3}{x}$

= $3x^{-1}$

=> f'(x) = $-3x^{-2}$

$\text{f'(x)}=$ $-\frac{3}{x^2}$