1/ Tìm x biết:a) \(2x + \frac{5}{2} =  - 1,5\)                          &nb

* Hướng dẫn giải

1a)

\(\begin{array}{*{20}{l}}
{2x - \frac{5}{2} = 1,5}\\
{2x = \frac{3}{2} + \frac{5}{2}}\\
{2x = 4}\\
{x = 2}
\end{array}\)

1b)

\(\begin{array}{*{20}{l}}
{\frac{2}{3} - \frac{1}{3}x = \frac{5}{6}}\\
{{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} \frac{1}{3}x = \frac{2}{3} - \frac{5}{6}}\\
{{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} \frac{1}{3}x = \frac{{ - 1}}{6}}\\
{{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\mkern 1mu} x = \frac{{ - 1}}{6}:\frac{1}{3} = \frac{{ - 1}}{2}}
\end{array}\)

2) \(\frac{{91}}{{1.4}} + \frac{{91}}{{4.7}} + \frac{{91}}{{7.10}} + ........... + \frac{{91}}{{88.91}}\)

\(\begin{array}{l}
 = \frac{{91}}{3}.\left( {\frac{3}{{1.4}} + \frac{3}{{4.7}} + \frac{3}{{7.10}} + ........... + \frac{3}{{88.91}}} \right)\\
 = \frac{{91}}{3}.\left( {\frac{1}{1} - \frac{1}{4} + \frac{1}{4} - \frac{1}{7} + \frac{1}{7} - \frac{1}{{10}} + ........... + \frac{1}{{88}} - \frac{1}{{91}}} \right)\\
 = \frac{{91}}{3}.\left( {\frac{1}{1} - \frac{1}{{91}}} \right) = \frac{{91}}{3}.\frac{{90}}{{91}} = 30
\end{array}\)