Tìm giá trị của x , biết: (a) 4/5 − x = 6/15 . (b) 0 , 6 x + 4/5 ( x − 10 ) = − 7 .

a) \(\frac{4}{5} - x = \frac{6}{{15}}\)

\(x = \frac{4}{5} - \frac{6}{{15}}\)

\(x = \frac{2}{5}\)

Vậy \(x = \frac{2}{5}\).

b) \(0,6x + \frac{4}{5}\left( {x - 10} \right) = - 7\)

\(\frac{3}{5}x + \frac{4}{5}x - 8 = - 7\)

\(\left( {\frac{3}{5} + \frac{4}{5}} \right)x = - 7 + 8\)

\(\frac{7}{5}x = 1\)

\(x = 1:\frac{7}{5}\)

\(x = \frac{5}{7}\)

Vậy \(x = \frac{5}{7}\).

c) \(0,5 - \frac{3}{4}{x^2} = \frac{{ - 5}}{2}\)

\(\frac{1}{2} - \frac{3}{4}{x^2} = \frac{{ - 5}}{2}\)

\(\frac{3}{4}{x^2} = \frac{1}{2} - \frac{{ - 5}}{2}\)

\(\frac{3}{4}{x^2} = 3\)

\({x^2} = 3:\frac{3}{4}\)

\({x^2} = 4\)

\(x = \pm 2\)

Vậy \(x \in \left\{ { \pm 2} \right\}\).