Tìm x biết: a) -5/12:x = 4/15; b) 3x/4 - 2x/3 = 2/7.1/6 + 5/7.1/6; c) 4x – (3 + 5x) = 14.

Hướng dẫn giải

a) \[\frac{{ - 5}}{{12}}:x = \frac{4}{{15}}\]

\[x = \frac{{ - 5}}{{12}}:\frac{4}{{15}}\]

\[x = \frac{{ - 5}}{{12}}.\frac{{15}}{4}\]

\(x = \frac{{ - 5.15}}{{12.4}}\)

\(x = \frac{{ - 5.3.5}}{{3.4.4}}\)

\[x = - \frac{{25}}{{16}}.\]

Vậy \[x = \frac{{ - 25}}{{16}}\].

b) \[\frac{3}{4}x - \frac{2}{3}x = \frac{2}{7}.\frac{1}{6} + \frac{5}{7}.\frac{1}{6}\]

\[\left( {\frac{3}{4} - \frac{2}{3}} \right)x = \frac{1}{6}.\frac{2}{7} + \frac{1}{6}.\frac{5}{7}\]

\[\left( {\frac{9}{{12}} - \frac{8}{{12}}} \right)x = \frac{1}{6}\left( {\frac{2}{7} + \frac{5}{7}} \right)\]

\(\frac{1}{{12}}.x = \frac{1}{6}.\frac{7}{7}\)
\[\frac{1}{{12}}x = \frac{1}{6}\]

\[x = \frac{1}{6}:\frac{1}{{12}}\]

\(x = \frac{1}{6}.\frac{{12}}{1}\)

x = 2.

Vậy x = 2.

c) 4x – (3 + 5x) = 14

4x – 3 – 5x = 14

4x – 5x = 14 + 3

(4 – 5).x = 17

–x = 17

x = –17.

Vậy x = –17.