Tính hợp lí: $A=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}$.

1) \[ - 0,5 + \frac{3}{2} = \frac{{ - 1}}{2} + \frac{3}{2}\]  

                   \[ = 1\]

 2) \({\left( { - 0,25} \right)^4}{.4^4}\) =  (-0,25.4)4  

                          = (-1)4 = 1

 3) \(\frac{2}{9}.\frac{8}{3} - \frac{{11}}{9}.\frac{8}{3} = \frac{8}{3}.\left( {\frac{2}{9} - \frac{{11}}{9}} \right)\)  

                        \( = \frac{8}{3}.( - 1) =  - \frac{8}{3}\)

 4) \(\frac{{14}}{5} + \left( {2,5 - \frac{9}{5}} \right) = \frac{{14}}{5} - \frac{9}{5} + 2,5\)

                             \( = 1 + 2,5\)\( = 3,5\)

1) \[{\left( {\frac{3}{2}} \right)^3}.x = {\left( {\frac{3}{2}} \right)^5}\]

 \[x = {\left( {\frac{3}{2}} \right)^5}:{\left( {\frac{3}{2}} \right)^3}\]

 \[x = {\left( {\frac{3}{2}} \right)^2}\]

 \[x = \frac{9}{4}\]  

 2) \[\frac{1}{2} - x = \frac{3}{4}\]

 \(x = \frac{1}{2} - \frac{3}{4}\)

 \(x = \frac{2}{4} - \frac{3}{4}\)

 \(x = \frac{{ - 1}}{4}\)

 3) \[{\left( {x - 0,3} \right)^2} = 9\]

 \[{\left( {x - 0,3} \right)^2} = {3^2}\] hoặc \[{\left( {x - 0,3} \right)^2} = {( - 3)^2}\]

 \(x - 0,3 = 3\) hoặc \(x - 0,3 =  - 3\)

 \(x = 3,3\) hoặc  \[x =  - 2,7\]

 4)  \[\frac{{ - 27}}{8}:x = {\left( {\frac{3}{2}} \right)^3}\]

 \[\frac{{ - 27}}{8}:x = \frac{{27}}{8}\]

 \[x = \frac{{ - 27}}{8}:\frac{{27}}{8}\]

 \[x =  - 1\]