Giải bài 2 trang 43 SGK Toán 6 Cánh diều

Đề bài

Tìm số thích hợp cho [?]:

a) $\frac{{ - 2}}{3}.\frac{{\left[ ? \right]}}{4} = \frac{1}{2}$;

b) $\frac{{\left[ ? \right]}}{3}.\frac{5}{8} = \frac{{ - 5}}{{12}}$;

c) $\frac{5}{6}.\frac{3}{{\left[ ? \right]}} = \frac{1}{4}.$

Lời giải chi tiết

a)

$\begin{array}{l}\frac{{ - 2}}{3}.\frac{{\left[ ? \right]}}{4} = \frac{1}{2}\\\frac{{\left[ ? \right]}}{4} = \frac{1}{2}:\left( {\frac{{ - 2}}{3}} \right)\\\frac{{\left[ ? \right]}}{4} = \frac{1}{2}.\frac{{ - 3}}{2}\\\frac{{\left[ ? \right]}}{4} = \frac{{ - 3}}{4}\\ \Rightarrow \left[ ? \right] =  - 3\end{array}$

b)

$\begin{array}{l}\frac{{\left[ ? \right]}}{3}.\frac{5}{8} = \frac{{ - 5}}{{12}}\\\frac{{\left[ ? \right]}}{3} = \frac{{ - 5}}{{12}}:\frac{5}{8}\\\frac{{\left[ ? \right]}}{3} = \frac{{ - 5}}{{12}}.\frac{8}{5}\\\frac{{\left[ ? \right]}}{3} = \frac{{ - 2}}{3}\\ \Rightarrow \left[ ? \right] =  - 2\end{array}$

c)

 $\begin{array}{l}\frac{5}{6}.\frac{3}{{\left[ ? \right]}} = \frac{1}{4}\\\frac{3}{{\left[ ? \right]}} = \frac{1}{4}:\frac{5}{6}\\\frac{3}{{\left[ ? \right]}} = \frac{1}{4}.\frac{6}{5}\\\frac{3}{{\left[ ? \right]}} = \frac{3}{{10}}\\ \Rightarrow \left[ ? \right] = 10\end{array}$

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