Cho hình vẽ. Biết \(DE\perp AB\), \(AC\perp AB\). Tìm x?
Do \(DE\perp AB\), \(AC\perp AB\) \(\Rightarrow\) \(DE\)//\(AC\)
Theo định lí Talet ta có: \(\dfrac{BD}{DA}=\dfrac{BE}{EC}\)
\(\Rightarrow\dfrac{6}{x}=\dfrac{3x}{13,5-3x}\)
\(\Rightarrow x^2+6x-27=0\)
\(\Rightarrow\left(x-3\right)\left(x+9\right)=0\) \(\Rightarrow\left[{}\begin{matrix}x=3\\x=-9\left(L\right)\end{matrix}\right.\)
