Giải bài 5 trang 43

Đề bài

Tìm số thích hợp cho [?]:

a) $\frac{3}{16}:\frac{[?]}{8}=\frac{3}{4}$;

b) $\frac{1}{25}:\frac{-3}{[?]}=\frac{-1}{15}$;

c) $\frac{[?]}{12}:\frac{-4}{9}=\frac{-3}{16}$.

Lời giải chi tiết

a)

 $\begin{array}{c}\frac{3}{16}:\frac{[?]}{8}=\frac{3}{4}\\ \frac{[?]}{8}=\frac{3}{16}:\frac{3}{4}\\ \frac{[?]}{8}=\frac{3}{16}.\frac{4}{3}\\ \frac{[?]}{8}=\frac{12}{48}\\ \frac{[?]}{8}=\frac{2}{8}\\ \Rightarrow [?]=2\end{array}$

b)

$\begin{array}{c}\frac{1}{25}:\frac{-3}{[?]}=\frac{-1}{15}\\ \frac{-3}{[?]}=\frac{1}{25}:\frac{-1}{15}\\ \frac{-3}{[?]}=\frac{1}{25}.\frac{-15}{1}\\ \frac{-3}{[?]}=\frac{-15}{25}\\ \frac{-3}{[?]}=\frac{-3}{5}\\ \Rightarrow [?]=5\end{array}$

c)

$\begin{array}{c}\frac{[?]}{12}:\frac{-4}{9}=\frac{-3}{16}\\ \frac{[?]}{12}=\frac{-3}{16}.\frac{-4}{9}\\ \frac{[?]}{12}=\frac{12}{144}\\ \frac{[?]}{12}=\frac{1}{12}\\ \Rightarrow [?]=1\end{array}$

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