Hình a:
Do $DE//BC$, theo Talets, ta có:
$\dfrac{AD}{DB}=\dfrac{AE}{EC}$
Do: $\dfrac{AD}{DB}=\dfrac{3,5}{2}$
$\to \dfrac{AE}{EC}=\dfrac{5}{x}=\dfrac{3,5}{2}$
$\to x=\dfrac{20}{7}$
Hình b:
Do $\widehat{DKI}=\widehat{DFE}$
$\to IK//EF$ (do đồng vị)
Theo Talets, ta có:
$\dfrac{DI}{IE}=\dfrac{DK}{KF}$
Mà: $\dfrac{DK}{KF}=\dfrac{2,4}{6}$
$\to \dfrac{DI}{IE}=\dfrac{DI}{5}=\dfrac{2,4}{6}$
$\to DI=2$
Do: $x=DI+IE=2+5=7$
