Tìm x, biết: a) 3,14 - x = 8,34;  b) x/14 =- 81/21

a) \[3,14 - x = 8,34\]

    \[x = 3,14 - 8,34\]

    \[x =  - 5,2\]

Vậy \[x =  - 5,2\].    

b) \(\frac{x}{{14}} = \frac{{ - 81}}{{21}}\)

\(x \cdot 21 =  - 81 \cdot 14\)

\(x \cdot 21 =  - 1134\)

\(x =  - 1134:21\)

\(x =  - 54\)

Vậy \(x =  - 54\).

c) \[\frac{1}{2}{x^2} - 4\frac{1}{2} = 0\]

\[\frac{1}{2}{x^2} = \frac{9}{2}\]

\[{x^2} = \frac{9}{2}:\frac{1}{2}\]

\({x^2} = \frac{9}{2}.\frac{2}{1}\)

\({x^2} = 9\)

Suy ra \[x = 3\] hoặc \[x =  - 3\]

Vậy \(x \in \left\{ {3; - 3} \right\}\).