Giải bài 2 trang 43

Đề bài

Tìm số thích hợp cho [?]:

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

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

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

Lời giải chi tiết

a)

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

b)

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

c)

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

trithuchub.com