$$f\left(x\right)=\frac{x^2}{x^4+16}$$

$$f\left(x\right)=2x-4$$

$$f\left(x\right)=\frac{1}{x\sqrt{x}}$$

$$f\left(x\right)=\left(4x+1\right)\left(6x+3\right)$$

$$f\left(x\right)=\frac{1}{\left(1+x\right)^2}$$

$$f\left(x\right)=x^2-4x$$

$$f\left(x\right)=x^{\frac{4}{3}}$$