Tìm x a) -5/6 . 78/25 + x = 4/15 . -3/8 b) 3/4 . 7/9 + 1/4 . 7/9 -x =1

* Hướng dẫn giải

a) 

$\begin{array}{l}-\frac{5}{6}.\frac{78}{25}+x=\frac{4}{15}.\frac{-3}{8}\\ \frac{-13}{5}+x=\frac{-1}{10}\\ x=\frac{-1}{10}-\frac{-13}{5}\\ x=\frac{5}{2}\end{array}$

b)

$\begin{array}{l}\frac{3}{4}.\frac{7}{9}+\frac{1}{4}.\frac{7}{9}-x=1\\ \frac{7}{9}\left(\frac{3}{4}+\frac{1}{4}\right)-x=1\\ \frac{7}{9}-x=1\\ x=\frac{-2}{9}\end{array}$