Đáp án:
a) $\frac{{AD}}{{AB}} = \frac{3}{4}$
b) Theo định lý Ta lét ta có: DE//BC thì:
$\frac{{DE}}{{BC}} = \frac{{AD}}{{AB}} = \frac{3}{4}$
c)
$\begin{array}{l}
\frac{{DE}}{{BC}} = \frac{3}{4}\\
\Rightarrow BC = \frac{{4DE}}{3} = \frac{{4.0,75}}{3} = 1\left( {cm} \right)
\end{array}$
Vậy BC =1cm