Tính hợp lí: A = 5/ 2.1 + 4/ 1.11 + 3/ 11.2
Giải thích
\[A = \frac{5}{{2.1}} + \frac{4}{{1.11}} + \frac{3}{{11.2}} + \frac{1}{{2.15}} + \frac{{13}}{{15.4}}\]
\[\,\,\,\,\, = 7.\left( {\frac{5}{{2.7}} + \frac{4}{{7.11}} + \frac{3}{{11.14}} + \frac{1}{{14.15}} + \frac{{13}}{{15.28}}} \right)\]
\[ = 7.\left( {\frac{1}{2} - \frac{1}{7} + \frac{1}{7} - \frac{1}{{14}} + \frac{1}{{11}} - \frac{1}{{14}} + \frac{1}{{14}} - \frac{1}{{15}} + \frac{1}{{15}} - \frac{1}{{28}}} \right)\]
\[ = 7.\left( {\frac{1}{2} - \frac{1}{{28}}} \right)\]
\[ = 7.\frac{{13}}{{28}} = \frac{{13}}{4}\]