` x + (x + 1) + (x + 2) + ... + 2018 + 2019 = 2019 `
` => x + (x + 1) + (x + 2) + ... + 2018 = 0 `
` => (2019 - x).\frac{2018+x}{2} = 0 `
` => ` \(\left[ \begin{array}{l}2019-x=0\\2018+x=0\end{array} \right.\)
` => ` \(\left[ \begin{array}{l}x=2019(loại)\\x=-2018\end{array} \right.\)
Vậy ` x = -2018 `