\(\displaystyle a)\;{{11} \over {12}} - \left( {{2 \over 5} + x} \right) = {2 \over 3} \)
\(\displaystyle \left( {{2 \over 5} + x} \right) = {{11} \over {12}} - {2 \over 3}\)
\( \displaystyle {2 \over 5} + x = {{11} \over {12}} - {8 \over {12}}\)
\(\displaystyle {2 \over 5} + x = {1 \over 4}\)
\(\displaystyle x = {1 \over 4} - {2 \over 5} \)
\(\displaystyle x = {5 \over {20}} - {8 \over {20}} \)
\(\displaystyle x = - {3 \over {20}}\)
\(\displaystyle b)\;2{\rm{x}}.\left( {x - {1 \over 7}} \right) = 0\)
\(\Rightarrow 2{\rm{x}} = 0\) hoặc \(\displaystyle {\rm{x}} - {1 \over 7} = 0\)
\(\Rightarrow x = 0\) hoặc \(\displaystyle x = {1 \over 7}\).
Vậy \(x = 0\) hoặc \(\displaystyle x = {1 \over 7}\)
\(\displaystyle c)\;{3 \over 4} + {1 \over 4}:x = {2 \over 5}\)
\(\displaystyle {1 \over 4}:x = {2 \over 5} - {3 \over 4}\)
\(\displaystyle {1 \over 4}:x = {8 \over {20}} - {{15} \over {20}}\)
\(\displaystyle {1 \over 4}:x = {{ - 7} \over {20}}\)
\(\displaystyle x = {1 \over 4}:{{ - 7} \over {20}} \)
\(\displaystyle x = {1 \over 4}.{{ - 20} \over 7} = {{ - 5} \over 7}\)
