Đáp án:
$\begin{array}{l}
3a)\frac{{AB}}{{AC}} = \frac{6}{{18}} = \frac{1}{3}\\
b)\frac{{AB}}{{CD}} = \frac{7}{{14}} = \frac{1}{2}\\
\frac{{MN}}{{PQ}} = \frac{{20}}{{10}} = 2\\
4)\\
a)Theo\,Talet:\frac{{AM}}{{MB}} = \frac{{AN}}{{NC}}\\
\Rightarrow \frac{{7,5}}{5} = \frac{x}{8}\\
\Rightarrow x = 8.1,5 = 12\left( {cm} \right)\\
b)Theo\,Talet:\frac{{BC}}{{CD}} = \frac{{AC}}{{CE}} = \frac{{AB}}{{DE}}\\
\Rightarrow \frac{x}{5} = \frac{{3,5}}{y} = \frac{3}{6} = \frac{1}{2}\\
\Rightarrow \left\{ \begin{array}{l}
x = 2,5\\
y = 7
\end{array} \right.\\
c)Theo\,t/c\,tia\,phân\,giác:\frac{{BD}}{{AB}} = \frac{{CD}}{{AC}}\\
\Rightarrow \frac{x}{8} = \frac{{CD}}{{12}} = \frac{{x + CD}}{{8 + 12}} = \frac{{BC}}{{20}} = \frac{1}{2}\\
\Rightarrow x = 4
\end{array}$
