QF = 24 - 9 = 15
ta có: PQ // EF
\(\Leftrightarrow\dfrac{DP}{EP}=\dfrac{DQ}{FQ}\)
\(\Leftrightarrow\dfrac{x}{10,5}=\dfrac{9}{15}\)
\(\Leftrightarrow15x=10,5.9\)
\(\Leftrightarrow15x=94,5\Leftrightarrow x=6,3\)
Xét ΔDEF có PQ//EF
nên \(\dfrac{x}{x+10.5}=\dfrac{9}{24}=\dfrac{3}{8}\)
=>8x=3x+31,5
=>5x=31,5
hay x=6,3(cm)
\(QF=DF-DQ=24-9=15.\)
Xét \(\Delta DEF:\)
\(PQ//EF\left(gt\right).\\ \Rightarrow\dfrac{DP}{PE}=\dfrac{DQ}{QF}\left(Telet\right).\)
\(Thay:\)\(\dfrac{x}{10,5}=\dfrac{9}{15}.\Rightarrow x=6,3.\)
