Đề bài

Tìm số nguyên thích hợp cho ô vuông: a) \(\frac{7}{{25}}.\frac{{?}}{{28}} = \frac{{ - 3}}{{20}};\) b) \(\frac{{46}}{{15}}.\frac{{ - 3}}{{?}} = \frac{{23}}{5};\) c) \(\frac{{?}}{{ - 18}}.\frac{5}{2} = \frac{{ - 5}}{{12}}\)

Lời giải chi tiết

\(a)\frac{7}{{25}}.\frac{?}{{28}} = \frac{{ - 3}}{{20}} \Leftrightarrow \frac{?}{{28}} = \frac{{ - 3}}{{20}}:\frac{7}{{25}} \\\Leftrightarrow \frac{?}{{28}} = \frac{{ - 3}}{{20}}.\frac{{25}}{7} \Leftrightarrow \frac{?}{{28}} = \frac{{ - 15}}{{28}}\\ \Leftrightarrow ?.28 = ( - 15).28 \Leftrightarrow ? =  - 15\)\(b)\frac{{46}}{{15}}.\frac{{ - 3}}{?} = \frac{{23}}{5} \Leftrightarrow \frac{{ - 3}}{?} = \frac{{23}}{5}:\frac{{46}}{{15}} \\ \Leftrightarrow \frac{{ - 3}}{?} = \frac{{23}}{5}.\frac{{15}}{{46}} \Leftrightarrow \frac{{ - 3}}{?} = \frac{3}{2} \\ \Leftrightarrow ( - 3).2 = 3.? \Leftrightarrow ? =  - 2\)\(c)\frac{?}{{ - 18}}.\frac{5}{2} = \frac{{ - 5}}{{12}} \Leftrightarrow \frac{?}{{ - 18}} = \frac{{ - 5}}{{12}}:\frac{5}{2}\\ \Leftrightarrow \frac{?}{{ - 18}} = \frac{{ - 5}}{{12}}.\frac{2}{5} \Leftrightarrow \frac{?}{{ - 18}} = \frac{{ - 1}}{6}\\ \Leftrightarrow ?.6 = ( - 1).( - 18) \Leftrightarrow ? = 3\)