Giải bài 6 trang 43

Đề bài

Tìm x, biết:

a) $\frac{4}{7}.x-\frac{2}{3}=\frac{1}{5}$;

b) $\frac{4}{5}+\frac{5}{7}:x=\frac{1}{6}$.

Lời giải chi tiết

a)

$\begin{array}{c}\frac{4}{7}.x-\frac{2}{3}=\frac{1}{5}\\ \frac{4}{7}.x=\frac{1}{5}+\frac{2}{3}\\ \frac{4}{7}.x=\frac{3}{15}+\frac{10}{15}\\ \frac{4}{7}.x=\frac{13}{15}\\ x=\frac{13}{15}:\frac{4}{7}\\ x=\frac{13}{15}.\frac{7}{4}\\ x=\frac{91}{60}\end{array}$

Vậy $x=\frac{91}{60}$.

b)

$\begin{array}{c}\frac{4}{5}+\frac{5}{7}:x=\frac{1}{6}\\ \frac{5}{7}:x=\frac{1}{6}-\frac{4}{5}\\ \frac{5}{7}:x=\frac{5}{30}-\frac{24}{30}\\ \frac{5}{7}:x=\frac{-19}{30}\\ x=\frac{5}{7}:\frac{-19}{30}\\ x=\frac{5}{7}.\frac{-30}{19}\\ x=\frac{-150}{133}\end{array}$

Vậy $x=\frac{-150}{133}$. 

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