Tìm 1 + 2 + 3 + + x = 1275
Tìm 1 + 2 + 3 + …. + x = 1275.
1 + 2 + 3 + …. + x = 1275
⇔ (1 + x) . x : 2 = 1275
⇔ x2 + x = 2550
⇔ x2 + x – 2550 = 0
⇔ \[\left[ \begin{array}{l}x = 50\\x = - 51\end{array} \right.\]
Chọn x = 50.