Question

Answer 1

Xét ΔDEF có MN//EF

nên 3/x=4/12=1/3

=>x=9

Answer 2

\(DF=DN+NF=4+8=12\left(cm\right)\)

\(\text{Xét }\Delta DMN\text{ và }\Delta DEF\text{ có:}\)

\(MN//EF\left(gt\right)\)

\(\Rightarrow\dfrac{DN}{DF}=\dfrac{MN}{EF}\left(\text{hệ quả Ta lét}\right)\)

\(\Rightarrow\dfrac{4}{12}=\dfrac{3}{x}\)

\(\Rightarrow x=\dfrac{12.3}{4}=9\left(cm\right)\)