Đáp án:
$\begin{array}{l}
3.5\\
a)\dfrac{{ - 6}}{x} = \dfrac{{ - 8}}{{12}}\\
\Rightarrow x.\left( { - 8} \right) = \left( { - 6} \right).12\\
\Rightarrow x = \dfrac{{6.12}}{8} = 9\\
Vậy{\kern 1pt} x = 9\\
b)\dfrac{x}{{ - 7}} = \dfrac{{14}}{{49}}\\
\Rightarrow x.49 = \left( { - 7} \right).14\\
\Rightarrow x = \dfrac{{ - 7.14}}{{49}} = 2\\
Vậy{\kern 1pt} x = 2\\
3.6\\
\dfrac{4}{x} = \dfrac{y}{{21}} = \dfrac{z}{{49}} = \dfrac{{52}}{{91}}\\
\Rightarrow \left\{ \begin{array}{l}
x = \dfrac{{4.91}}{{52}} = 7\\
y = \dfrac{{21.52}}{{91}} = 12\\
z = \dfrac{{49.52}}{{91}} = 28
\end{array} \right.\\
Vậy{\kern 1pt} x = 7;y = 12;z = 28\\
3.7\\
a)Do:\dfrac{{ - 21}}{{28}} = \dfrac{{ - 21:7}}{{28:7}} = \dfrac{{ - 3}}{4}\\
\dfrac{{ - 39}}{{52}} = \dfrac{{ - 39:13}}{{52:13}} = \dfrac{{ - 3}}{4}\\
\Rightarrow \dfrac{{ - 21}}{{28}} = \dfrac{{ - 39}}{{52}}\\
b)\dfrac{{ - 1313}}{{2121}} = \dfrac{{ - 1313:101}}{{2121:101}} = \dfrac{{ - 13}}{{21}}\\
\dfrac{{ - 131313}}{{212121}} = \dfrac{{ - 131313:10101}}{{212121:10101}} = \dfrac{{ - 13}}{{21}}\\
\Rightarrow \dfrac{{ - 1313}}{{2121}} = \dfrac{{ - 131313}}{{212121}}\\
3.8\\
a)\dfrac{{482 - 39}}{{567 - 28}}\\
\dfrac{{964 - 78}}{{1134 - 56}} = \dfrac{{964:2 - 78:2}}{{1134:2 - 56:2}} = \dfrac{{482 - 39}}{{567 - 28}}\\
b)\dfrac{{4563 - 213}}{{711 - 51}} = \dfrac{{4563:3 - 213:3}}{{711:3 - 51:3}}\\
= \dfrac{{1521 - 71}}{{237 - 17}}\\
3.10\\
\dfrac{{ - 9}}{{39}} = \dfrac{{ - 9:3}}{{39:3}} = \dfrac{{ - 3}}{{13}}\\
\dfrac{{12}}{9} = \dfrac{{12:3}}{{9:3}} = \dfrac{4}{3}\\
\dfrac{{ - 35}}{{10}} = \dfrac{{ - 35:5}}{{10:5}} = \dfrac{{ - 7}}{2}
\end{array}$
