x2 + 7x + 8 = 0

x2 + 7x = 0 - 8

x2 + 7x = -8

Đặt \(\left(\dfrac{b}{2}\right)^2=\left(\dfrac{7}{2}\right)^2\)

\(\Rightarrow\) x2 + 7x + \(\left(\dfrac{7}{2}\right)^2\) = ( -8 ) + \(\left(\dfrac{7}{2}\right)^2\)

\(\Rightarrow\) x2 + 7x + \(\left(\dfrac{7}{2}\right)^2\) = ( -8 ) + \(\dfrac{49}{4}\)

\(\Rightarrow\) x2 + 7x + \(\dfrac{49}{4}=\dfrac{17}{4}\)

\(\Rightarrow\) ( x + \(\dfrac{7}{2}\) )2 = \(\dfrac{17}{4}\)

( x + \(\dfrac{7}{2}\) )2.1/2 = \(\pm\sqrt{\dfrac{81}{4}}\)

( x + \(\dfrac{7}{2}\) ) = \(\pm\) \(\sqrt{\dfrac{81}{4}}\)

( x + \(\dfrac{7}{2}\) ) = \(\pm\) \(\dfrac{\sqrt{81}}{\sqrt{4}}\)

( x + \(\dfrac{7}{2}\) ) = \(\pm\) \(\dfrac{9}{2}\)

\(\left\{{}\begin{matrix}x+\dfrac{7}{2}=\dfrac{9}{2}\\x+\dfrac{7}{2}=\dfrac{-9}{2}\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{9}{2}-\dfrac{7}{2}\\x=\dfrac{-9}{2}-\dfrac{7}{2}\end{matrix}\right.\)

\(\Rightarrow\) \(\left\{{}\begin{matrix}x=1\\x=-8\end{matrix}\right.\)

Vậy x = 1 hoặc x = -8