Tìm giá trị của x x, biết: a) x + 1,05 = 4,25 - 0,2; b) 1/3 + (2/3) x = 7,5

a) \[x + 1,05 = 4,25 - 0,2\]

\[x + 1,05 = 4,05\]

\[x = 4,05 - 1,05\]

\[x = 3\].

Vậy \[x = 3\].

b) \(\frac{1}{3} + \frac{2}{3}x = 7,5\)

\(\frac{2}{3}x = \frac{{15}}{2} - \frac{1}{3}\)

\(\frac{2}{3}x = \frac{{43}}{6}\)

\(x = \frac{{43}}{6}:\frac{2}{3}\)

\(x = \frac{{43}}{6}.\frac{3}{2}\)

\(x = \frac{{43}}{4}\)

Vậy \(x = \frac{{43}}{4}\).

c) \(\left( {4x + 5} \right)\left( {\frac{5}{4}x - 2} \right) = 0\)

Trường hợp 1:

\(4x + 5 = 0\)

\(4x =  - 5\)

\(x =  - \frac{5}{4}\)

 Vậy\(x \in \left\{ { - \frac{5}{4};\frac{8}{5}} \right\}\).

Trường hợp 2:

\(\frac{5}{4}x - 2 = 0\)

\(\frac{5}{4}x = 2\)

\[x = 2:\frac{5}{4}\]

\[x = \frac{8}{5}\]