x.(x+1) = 2 + 4 + 6 + 8 +10

$x$.($x+1$) = $2 + 4 + 6 + 8 +10$

⇔$x$.($x+1$) = $30$

⇒\(\left[ \begin{array}{l}x(x+1)=5.6\\x(x+1)=-6.(-5)\end{array} \right.\) 

⇔\(\left[ \begin{array}{l}x=5\\x=-6\end{array} \right.\) 

   Vây $x$ ∈ {$-6;5$}