a) Ta có: \(\dfrac{3}{5\cdot7}+\dfrac{3}{7\cdot9}+...+\dfrac{3}{59\cdot61}\)
\(=\dfrac{3}{2}\left(\dfrac{2}{5\cdot7}+\dfrac{2}{7\cdot9}+...+\dfrac{2}{59\cdot61}\right)\)
\(=\dfrac{3}{2}\left(\dfrac{1}{5}-\dfrac{1}{61}\right)\)
\(=\dfrac{3}{2}\cdot\dfrac{56}{305}=\dfrac{84}{305}\)
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a)
3/5.7+3/7.9+...+3/59.61
=3/2.(2/5.7+2/7.9+...+2/59.61)
=3/2.(1/5−1/7+1/7−1/9+...+1/59−1/61)
=3/2(1/5−1/61)
=3/2.56/305
=844/305
b) câu b, bạn vt hơi khó hiểu
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