Tìm x, biết rằng: 5/8 x +2/5 = 1/5 b) 5/7 : x +11/7 = 18/7

* Hướng dẫn giải

a) 

$\begin{array}{l}\frac{5}{8}x+\frac{2}{5}=\frac{1}{5}\\ \frac{5}{8}x=\frac{1}{5}-\frac{2}{5}\\ \frac{5}{8}x=-\frac{1}{5}\\ x=\frac{-1}{5}:\frac{5}{8}\\ x=\frac{-1}{5}.\frac{8}{5}\\ x=\frac{-8}{25}\end{array}$

b)

$\begin{array}{l}\frac{5}{7}:x+\frac{11}{7}=\frac{18}{7}\\ \frac{5}{7}:x=\frac{18}{7}-\frac{11}{7}\\ \frac{5}{7}:x=1\\ x=\frac{5}{7}:1\\ x=\frac{5}{7}\end{array}$

c)

$\begin{array}{l}\left(\frac{2x}{5}-1\right):\left(-5\right)=\frac{1}{7}\\ \left(\frac{2x}{5}-1\right)=\frac{1}{7}.\left(-5\right)\\ \frac{2x}{5}-1=\frac{-5}{7}\\ \frac{2x}{5}=\frac{-5}{7}+1\\ \frac{2x}{5}=\frac{2}{5}\\ x=\frac{2}{5}:\frac{2}{5}\\ x=1\end{array}$