1. Thực hiện phép tính (tính nhanh nếu có thể): a) 2/3 +(5/7 +- 2/3);2. Tìm x: a) x/14 =- 81/21; 

1.

a) \(\frac{2}{3} + \left( {\frac{5}{7} + \frac{{ - 2}}{3}} \right)\)\( = \frac{2}{3} + \frac{5}{7} + \frac{{ - 2}}{3}\)

\[ = \left( {\frac{2}{3} + \frac{{ - 2}}{3}} \right) + \frac{5}{7}\]\( = 0 + \frac{5}{7} = \frac{5}{7}\);

b) \(\frac{5}{{17}}.\frac{{ - 7}}{3} + \frac{8}{{17}}.\frac{{ - 7}}{3} + \frac{{ - 7}}{3}.\frac{4}{{17}}\)

\[ = \frac{{ - 7}}{3}.\left( {\frac{5}{{17}} + \frac{8}{{17}} + \frac{4}{{17}}} \right)\]

\[ = \frac{{ - 7}}{3}.1 =  - \frac{7}{3}\].

2.

a) \(\frac{x}{{14}} = \frac{{ - 81}}{{21}}\) \(x \cdot 21 =  - 81 \cdot 14\) \(x \cdot 21 =  - 1134\) \(x =  - 1134:21\) \(x =  - 54\) Vậy \(x =  - 54\).

b) \[\frac{1}{2}{x^2} - 4\frac{1}{2} = 0\] \[\frac{1}{2}{x^2} = \frac{9}{2}\] \[{x^2} = \frac{9}{2}:\frac{1}{2}\] \({x^2} = \frac{9}{2}.\frac{2}{1}\) \({x^2} = 9\) Suy ra \[x = 3\] hoặc \[x =  - 3\] Vậy \(x \in \left\{ {3; - 3} \right\}\).