1, $x+(x+1)+(x+2)+...+13+14=14$
$⇒x+(x+1)+...+13=0$
$⇒(13+x).$$\frac{13-x+1}{2}=0$
$⇒(13+x).(7-x)=0$
$⇒$\(\left[ \begin{array}{l}13+x=0\\7-x=0\end{array} \right.\)
$⇒$\(\left[ \begin{array}{l}x=-13\\x=7\end{array} \right.\)
Thử lại:
$x=-13(t/m)$
$x=7(loại)$
Vậy $x=-13$
2, $25+24+23+...+(x-3)=25$
$⇒24+23+...+(x-3)=0$
$⇒(x-3)+(x-2)+...+24=0$
$⇒(24+x-3).$$\frac{24-x+3+1}{2}=0$
$⇒(21+x).(11-x)=0$
$⇒$\(\left[ \begin{array}{l}21+x=0\\11-x=0\end{array} \right.\)
$⇒$\(\left[ \begin{array}{l}x=-21\\x=11\end{array} \right.\)
Thử lại:
$x=-21(t/m)$
$x=11(laoij)$
Vậy $x=-21$
