\((2x+9)^2=-18\)
\(\Longleftrightarrow 4x^2+36x+81 =-18\mid\) beidseitige Addition von \(18\)
\(\Longleftrightarrow 4x^2+36x+99 =0\mid\) beidseitige Division durch \(4\)
\(\Longleftrightarrow x^2+9x+24.75 =0\)
\(p-q-\)Formel anwenden
\(x_{1,2}=-\dfrac{9}{2}\pm\sqrt{\dfrac{9^2}{4}-24.75}=-\dfrac{9}{2}\pm\sqrt{-\dfrac{9}{2}}=-\dfrac{9}{2}\pm\dfrac{3}{\sqrt{2}}i\)
André