Đáp án:
Áp dụng t.c dãy tỉ số bằng nhau
$\begin{array}{l}
a)\frac{a}{2} = \frac{b}{5} = \frac{c}{8} = \frac{{2c}}{{16}}\\
= \frac{{a + b - 2c}}{{2 + 5 - 16}} = \frac{{30}}{{ - 9}} = \frac{{ - 10}}{3}\\
\Rightarrow \left\{ \begin{array}{l}
a = \frac{{ - 10}}{3}.2 = \frac{{ - 20}}{3}\\
b = \frac{{ - 10}}{3}.5 = \frac{{ - 50}}{3}\\
c = \frac{{ - 10}}{3}.8 = \frac{{ - 80}}{3}
\end{array} \right.\\
b)\frac{a}{4} = \frac{b}{5}\\
\Rightarrow \frac{a}{8} = \frac{b}{{10}} = \frac{c}{3} = \frac{{a + b - c}}{{8 + 10 - 3}} = \frac{{60}}{{15}} = 4\\
\Rightarrow \left\{ \begin{array}{l}
a = 32\\
b = 40\\
c = 12
\end{array} \right.
\end{array}$
