Đáp án: $\,a = - 42;b = - 28;c = - 20$
Giải thích các bước giải:
$\begin{array}{l}
2a = 3b \Leftrightarrow \dfrac{a}{3} = \dfrac{b}{2} \Leftrightarrow \dfrac{a}{{21}} = \dfrac{b}{{14}}\\
5b = 7c \Leftrightarrow \dfrac{b}{7} = \dfrac{c}{5} \Leftrightarrow \dfrac{b}{{14}} = \dfrac{c}{{10}}\\
\Leftrightarrow \dfrac{a}{{21}} = \dfrac{b}{{14}} = \dfrac{c}{{10}}\\
= \dfrac{{3a}}{{63}} = \dfrac{{7b}}{{98}} = \dfrac{{5c}}{{50}}\\
= \dfrac{{3a - 7b + 5c}}{{63 - 98 + 50}} = \dfrac{{ - 30}}{{15}} = - 2\\
\Leftrightarrow \left\{ \begin{array}{l}
a = - 2.21 = - 42\\
b = - 2.14 = - 28\\
c = - 2.10 = - 20
\end{array} \right.\\
Vậy\,a = - 42;b = - 28;c = - 20
\end{array}$