Tìm x, biết: (a) x/4=−1/3; (b) x+1/5=3/4.

a) \[\frac{x}{4} = \frac{{ - 1}}{3}\]

\[3x = - 4\]

\[x = \frac{{ - 4}}{3}\].

Vậy \[x = \frac{{ - 4}}{3}\].

b) \(\frac{{x + 1}}{5} = \frac{3}{4}\)

Suy ra \[4\left( {x + 10} \right) = 5 \cdot 3\]

\[4x + 40 = 15\]

\[4x = --25\]

\(x = - \frac{{25}}{4}\)

Vậy \(x = - \frac{{25}}{4}\).

c) \(\frac{{2x + 1}}{6} = \frac{{3 - x}}{9}\)

\(9\left( {2x + 1} \right) = 6\left( {3 - x} \right)\)

\(18x + 9 = 18 - 6x\)

\(18x + 6x = 18 - 9\)

\(24x = 9\)

\(x = \frac{3}{8}\).

Vậy \(x = \frac{3}{8}\).

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

\( - 5\left( {2 - x} \right) = 4\left( {x - 3} \right)\)

\( - 10 + 5x = 4x - 12\)

\(5x - 4x = - 12 + 10\)

\(x = - 2\)

Vậy \(x = - 2\).