Tìm số hữu tỉ x , biết: a) x + 1,25 =1/2

a)

 \[x + 1,25 = \frac{1}{2}\]

 \(x + \frac{5}{4} = \frac{1}{2}\)

 \(x = \frac{1}{2} - \frac{5}{4}\)

 \(x = \frac{2}{4} - \frac{5}{4}\)

\(x = \frac{{ - 3}}{4}\)

b) \[\begin{array}{l}\frac{3}{5}x - \frac{3}{4} = \frac{{ - 3}}{2}\\\frac{3}{5}x = \frac{{ - 3}}{2} + \frac{3}{4}\end{array}\]

 \(\begin{array}{l}\frac{3}{5}x = \frac{{ - 6}}{4} + \frac{3}{4}\\\frac{3}{5}x = \frac{{ - 3}}{4}\end{array}\)

 \(\begin{array}{l}x = \frac{{ - 3}}{4}:\frac{3}{5}\\x = \frac{{ - 3}}{4}.\frac{5}{3}\end{array}\)

 \(x = \frac{{ - 5}}{4}\).