Bài giải:

x² + x = 30 

$\Leftrightarrow $x² + x – 30 = 0

$\Leftrightarrow $x² + 6x – 5x – 30 = 0

$\Leftrightarrow $x(x + 6) – 5(x + 6) = 0

$\Leftrightarrow $(x – 5)(x + 6) = 0

$\Leftrightarrow \left[ \begin{align}& x-5=0 \\ & x+6=0 \\ \end{align} \right.$              $\Leftrightarrow \left[ \begin{align}& x=5 \\ & x=-6 \\ \end{align} \right.$

Mà: x  > 0 nên x = 5

Vậy: x = 5