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\(=\left[16\cdot\left(\dfrac{1}{13}-\dfrac{1}{20}+\dfrac{1}{20}-\dfrac{1}{27}+...+\dfrac{1}{62}-\dfrac{1}{69}\right)\right]:\dfrac{-112}{897}\)
\(=16\left(\dfrac{1}{13}-\dfrac{1}{69}\right)\cdot\dfrac{-897}{112}\)
\(=-\dfrac{897}{7}\cdot\dfrac{56}{897}=-8\)
Ta có: \(\left(\frac{112}{13.20}+\frac{112}{20.27}+...+\frac{112}{62.69}\right)\)
=\(\left(\frac{112}{13}-\frac{112}{20}+\frac{112}{20}-\frac{112}{27}+...+\frac{112}{62}-\frac{112}{69}\right)\)
112 là tử chung
=>\(112\cdot\left(\frac{1}{13}-\frac{1}{20}+\frac{1}{20}-\frac{1}{27}+...+\frac{1}{62}-\frac{1}{69}\right)\)
Rút gọn các phân số cho nhau, ta còn:
\(112.\left(\frac{1}{13}-\frac{1}{69}\right)\)=\(112\cdot\frac{56}{897}=\frac{6272}{897}\)(1)
Ta có: \(\left(\frac{-5}{9\cdot13}-\frac{7}{9\cdot25}-\frac{13}{19\cdot25}-\frac{31}{19\cdot69}\right)\)=\(\left(-\frac{24}{325}-\frac{122}{32775}\right)\)=\(\left(-\frac{1322}{17043}\right)\)(2)
(1);(2) => \(A=\)\(\frac{6272}{897}\)\(:\left(-\frac{1322}{17043}\right)\)\(\)\(=\)\(-\frac{59584}{661}\)
C
b)\(\frac{2.2014}{1+\frac{1}{1+2}+\frac{1}{1+2+3}+...+\frac{1}{1+2+3+...+2014}}\)
\(=\frac{2.2014}{1+\frac{1}{\frac{2.3}{2}}+\frac{1}{\frac{3.4}{2}}+...+\frac{1}{\frac{2014.2015}{2}}}\)
\(=\frac{2.2014}{2\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2014.2015}\right)}\)
\(=\frac{2014}{\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2014.2015}}\)
\(=\frac{2014}{1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2014}-\frac{1}{2015}}\)
\(=\frac{2014}{1-\frac{1}{2015}}=\frac{2014}{\frac{2014}{2015}}=2015\)
a)\(\frac{\frac{105}{13.20}+\frac{105}{20.27}+...+\frac{105}{62.69}}{\frac{5}{9.13}+\frac{7}{9.25}+\frac{13}{19.25}+\frac{31}{19.69}}\)
\(=\frac{\frac{105}{7}.\left(\frac{7}{13.20}+\frac{7}{20.27}+...+\frac{7}{62.69}\right)}{\left(\frac{9}{9.13}-\frac{4}{9.13}\right)+\left(\frac{16}{9.25}-\frac{9}{9.25}\right)+\left(\frac{19}{19.25}-\frac{6}{19.25}\right)+\left(\frac{50}{19.69}-\frac{19}{19.69}\right)}\)
\(=\frac{\frac{105}{7}.\left(\frac{1}{13}-\frac{1}{20}+\frac{1}{20}-\frac{1}{27}+...+\frac{1}{62}-\frac{1}{69}\right)}{\left(\frac{1}{13}-\frac{1}{9}+\frac{1}{13}\right)+\left(\frac{1}{9}-\frac{1}{25}-\frac{1}{25}\right)+\left(\frac{1}{25}-\frac{1}{19}+\frac{1}{25}\right)+\left(\frac{1}{19}-\frac{1}{69}-\frac{1}{69}\right)}\)
\(=\frac{\frac{105}{7}.\left(\frac{1}{13}-\frac{1}{69}\right)}{\frac{1}{13}+\frac{1}{13}-\frac{1}{69}-\frac{1}{69}}=\frac{\frac{105}{7}\left(\frac{1}{13}-\frac{1}{69}\right)}{2\left(\frac{1}{13}-\frac{1}{69}\right)}=\frac{\frac{105}{7}}{2}=\frac{15}{2}\)