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a) \(\dfrac{5}{1\cdot6}+\dfrac{5}{6\cdot11}+...+\dfrac{5}{26\cdot31}\)
\(=1-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{11}+...+\dfrac{1}{26}-\dfrac{1}{31}\)
\(=1-\dfrac{1}{31}\)
\(=\dfrac{30}{31}\)
b) \(\dfrac{4}{11\cdot16}+\dfrac{4}{16\cdot21}+...+\dfrac{4}{61\cdot66}\)
\(=\dfrac{4}{5}\cdot\left(\dfrac{5}{11\cdot16}+\dfrac{5}{16\cdot21}+...+\dfrac{5}{61\cdot66}\right)\)
\(=\dfrac{4}{5}\cdot\left(\dfrac{1}{11}-\dfrac{1}{16}+\dfrac{1}{16}-...+\dfrac{1}{61}-\dfrac{1}{66}\right)\)
\(=\dfrac{4}{5}\cdot\left(\dfrac{1}{11}-\dfrac{1}{66}\right)\)
\(=\dfrac{4}{5}\cdot\dfrac{5}{66}\)
\(=\dfrac{4}{66}\)
\(=\dfrac{2}{33}\)
a) A = 5²/(1.6) + 5²/(6.11) + ... + 5²/(26.31)
= 5.[5/(1.6) + 5/(6.11) + ...+ 5/(26.31)]
= 5.(1 - 1/6 + 1/6 - 1/11 + ... + 1/26 - 1/31)
= 5.(1 - 1/31)
= 5.30/31
= 150/31
b) B = 4/(11.16) + 4/(16.21) + ... + 4/(61.66)
= 4/5 .[5/(11.16) + 5/(16.21) + ... + 5/(61.66)]
= 4/5.(1/11 - 1/16 + 1/16 - 1/21 + ... + 1/61 - 1/66)
= 4/5.(1/11 - 1/66)
= 4/5 . 5/66
= 2/33
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\(1\frac{4}{25}+\frac{7}{21}-\frac{4}{25}+0,5+\frac{14}{21}\)
\(=\left(1\frac{4}{25}-\frac{4}{25}\right)+\left(\frac{7}{21}+\frac{14}{21}\right)+0,5\)
\(=1+0+0,5\)
\(=1,5\)
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\(\left(25-11^2\right)\left(25-10^2\right)\left(25-9^2\right)...\left(25-5^2\right)\left(25-4^2\right)\left(25-3^2\right)\left(25-2^2\right)\left(25-1^2\right)\)
\(=\left(25-11^2\right)\left(25-10^2\right)\left(25-9^2\right)...\left(25-25\right)\left(25-4^2\right)\left(25-3^2\right)\left(25-2^2\right)\left(25-1^2\right)\)
\(=\left(25-11^2\right)\left(25-10^2\right)\left(25-9^2\right)...0.\left(25-4^2\right)\left(25-3^2\right)\left(25-2^2\right)\left(25-1^2\right)\)
\(=0\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(25-11^2\right)\left(25-10^2\right)\left(25-9^2\right)...\left(25-5^2\right)\left(25-4^2\right)\left(25-3^2\right)\left(25-2^2\right)\left(25-1^2\right)\)
\(=\left(25-11^2\right)\left(25-10^2\right)\left(25-9^2\right)...\left(25-25\right)\left(25-4^2\right)\left(25-3^2\right)\left(25-2^2\right)\left(25-1^2\right)\)
\(=\left(25-11^2\right)\left(25-10^2\right)\left(25-9^2\right)....0.\left(25-4^2\right)\left(25-3^2\right)\left(25-2^2\right)\left(25-1^2\right)\)
\(=0\)