Tính: . 1/2+1/3+1/4

Lời giải:

\[A = \frac{{\frac{1}{2} + \frac{1}{3} + \frac{1}{4} + ... + \frac{1}{{2014}}}}{{\frac{{2013}}{1} + \frac{{2012}}{2} + \frac{{2011}}{3} + ... + \frac{1}{{2013}}}}\]

\[ = \frac{{\frac{1}{2} + \frac{1}{3} + \frac{1}{4} + ... + \frac{1}{{2014}}}}{{\left( {\frac{{2012}}{2} + 1} \right) + \left( {\frac{{2011}}{3} + 1} \right) + ... + \left( {\frac{1}{{2013}} + 1} \right) + 1}}\]

\[ = \frac{{\frac{1}{2} + \frac{1}{3} + \frac{1}{4} + ... + \frac{1}{{2014}}}}{{\frac{{2014}}{2} + \frac{{2014}}{3} + ... + \frac{{2014}}{{2013}} + \frac{{2014}}{{2014}}}}\]

\[ = \frac{{\frac{1}{2} + \frac{1}{3} + \frac{1}{4} + ... + \frac{1}{{2014}}}}{{2014 \cdot \left( {\frac{1}{2} + \frac{1}{3} + \frac{1}{4} + ... + \frac{1}{{2014}}} \right)}}\]

\[ = \frac{1}{{2014}}.\]