$\dfrac{(a + b)(a^2 - ab + b^2)}{(a^2b + ab^2)}$ in simplest form is equal to:

1. $\dfrac{a}{b} - 1 + \dfrac{b}{a}$

2. $\dfrac{a^3 - b^3}{a^2b + ab^2}$

3. $\dfrac{a^2 + b^2}{ab}$

4. none of these

Given,

$\Rightarrow \dfrac{(a + b)(a^2 - ab + b^2)}{(a^2b + ab^2)}\\[1em] \Rightarrow \dfrac{(a + b)(a^2 - ab + b^2)}{ab(a + b)}\\[1em] \Rightarrow \dfrac{(a^2 - ab + b^2)}{ab}\\[1em] \Rightarrow \dfrac{a^2}{ab} - \dfrac{ab}{ab} + \dfrac{b^2}{ab} \\[1em] \Rightarrow \dfrac{a}{b} - 1 + \dfrac{b}{a}$

Hence, option 1 is the correct option.