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Cho hai biểu thức:

A=25.7+57.12+712.19+919.28+1128.39+139.40A = \frac{2}{{5.7}} + \frac{5}{{7.12}} + \frac{7}{{12.19}} + \frac{9}{{19.28}} + \frac{{11}}{{28.39}} + \frac{1}{{39.40}}B=120+144+177+1119+1170B = \frac{1}{{20}} + \frac{1}{{44}} + \frac{1}{{77}} + \frac{1}{{119}} + \frac{1}{{170}}.

Chứng minh A>BA > B.

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Lời giải

Hướng dẫn giải:

Ta có: A=25.7+57.12+712.19+919.28+1128.39+139.40A = \frac{2}{{5.7}} + \frac{5}{{7.12}} + \frac{7}{{12.19}} + \frac{9}{{19.28}} + \frac{{11}}{{28.39}} + \frac{1}{{39.40}}

=1517+17112+112119+119128+128139+139140 = \frac{1}{5} - \frac{1}{7} + \frac{1}{7} - \frac{1}{{12}} + \frac{1}{{12}} - \frac{1}{{19}} + \frac{1}{{19}} - \frac{1}{{28}} + \frac{1}{{28}} - \frac{1}{{39}} + \frac{1}{{39}} - \frac{1}{{40}}

=15140 = \frac{1}{5} - \frac{1}{{40}}=840140=740 = \frac{8}{{40}} - \frac{1}{{40}} = \frac{7}{{40}};

B=120+144+177+1119+1170B = \frac{1}{{20}} + \frac{1}{{44}} + \frac{1}{{77}} + \frac{1}{{119}} + \frac{1}{{170}}

=240+288+2154+2238+2340 = \frac{2}{{40}} + \frac{2}{{88}} + \frac{2}{{154}} + \frac{2}{{238}} + \frac{2}{{340}}

=2.(15.8+18.11+111.14+114.17+117.20) = 2.\left( {\frac{1}{{5.8}} + \frac{1}{{8.11}} + \frac{1}{{11.14}} + \frac{1}{{14.17}} + \frac{1}{{17.20}}} \right)

=2.13.(35.8+38.11+311.14+314.17+317.20) = 2.\frac{1}{3}.\left( {\frac{3}{{5.8}} + \frac{3}{{8.11}} + \frac{3}{{11.14}} + \frac{3}{{14.17}} + \frac{3}{{17.20}}} \right)

=23.(1518+18111+111114+114117+117120) = \frac{2}{3}.\left( {\frac{1}{5} - \frac{1}{8} + \frac{1}{8} - \frac{1}{{11}} + \frac{1}{{11}} - \frac{1}{{14}} + \frac{1}{{14}} - \frac{1}{{17}} + \frac{1}{{17}} - \frac{1}{{20}}} \right)

=23.(15120) = \frac{2}{3}.\left( {\frac{1}{5} - \frac{1}{{20}}} \right)=23.(420120) = \frac{2}{3}.\left( {\frac{4}{{20}} - \frac{1}{{20}}} \right)=23.320 = \frac{2}{3}.\frac{3}{{20}}=110 = \frac{1}{{10}}.

Ta có: 740>440=110\frac{7}{{40}} > \frac{4}{{40}} = \frac{1}{{10}}.

Do đó A>BA > B.

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