Tìm x biết a) 1/3 + 2/3 : x = -7 b) 2/3 : x = 1,4 - 12/5 c) (4,5 -2x) . 1 4/7

* Hướng dẫn giải

a) $\frac{1}{3}+\frac{2}{3}:x=-7$

$\begin{array}{l}\iff \frac{2}{3}:x=-7-\frac{1}{3}\\ \iff \frac{2}{3}:x=\frac{-22}{3}\\ \iff x=\frac{2}{3}:\frac{-22}{3}\\ \iff x=\frac{-1}{11}\end{array}$

b) $\frac{2}{3}:x=1,4-\frac{12}{5}$

$\begin{array}{l}\iff \frac{2}{3}:x=\frac{-7}{5}\\ \iff x=\frac{2}{3}:\frac{-7}{5}\\ \iff x=\frac{-10}{21}\end{array}$

c) $\left(4,5-2x\right).1\frac{4}{7}=\frac{11}{14}$

$\begin{array}{l}\iff \left(4,5-2\mathrm{x}\right)=\frac{11}{14}:\frac{11}{7}\\ \iff \frac{9}{2}-2\mathrm{x}=\frac{1}{2}\\ \iff 2\mathrm{x}=4\\ \iff \mathrm{x}=2\end{array}$