Ta có: \(\frac{AB}{CD}=\frac{2}{3}\Rightarrow\frac{AB}{2}=\frac{CD}{3}\Leftrightarrow\frac{AB}{2.4}=\frac{CD}{3.4}\)
Và: \(\frac{CD}{EF}=\frac{4}{6}\Rightarrow\frac{CD}{4}=\frac{EF}{6}\Leftrightarrow\frac{CD}{4.3}=\frac{FE}{6.3}\)
\(\Rightarrow\frac{AB}{8}=\frac{CD}{12}=\frac{EF}{18}=\frac{AB+CD+EF}{8+12+18}=\frac{70}{38}=\frac{35}{19}\)
\(\Rightarrow\frac{AB}{8}=\frac{35}{19}\Rightarrow AB=\frac{35.8}{19}=\frac{280}{19}cm\)
\(\Rightarrow\frac{CD}{12}=\frac{35}{19}\Rightarrow CD=\frac{35.12}{19}=\frac{420}{19}cm\)
\(\Rightarrow\frac{FE}{18}=\frac{15}{19}\Rightarrow EF=\frac{35.18}{19}=\frac{630}{19}cm\)
Vậy ........................