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![](https://rs.olm.vn/images/avt/0.png?1311)
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\(B=4.\left(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{29.30}\right)\)
\(B=4.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{29}-\frac{1}{30}\right)\)
\(B=4.\left(1-\frac{1}{30}\right)\)
\(B=4.\frac{29}{30}\)
\(B=\frac{58}{15}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(6,8\cdot3+6,8\cdot2+13,6\cdot2+6,8\)
\(=6,8\cdot3+6,8\cdot2+6,8\cdot2\cdot2+6,8\)
\(=6,8\cdot3+6,8\cdot2+6,8\cdot4+6,8\)
\(=6,8\cdot\left(3+2+4+1\right)\)
\(=6,8\cdot10\)
\(=68\)
nha !!!
6,8x3+6,8x2+13,6x2+6,8
= 6,8x3+6,8x2+6,8x2x2+6,8
=6,8x(3+2+4+1)
=6,8x10
=68
![](https://rs.olm.vn/images/avt/0.png?1311)
#)Giải :
Đặt \(A=4-\frac{2}{1.2}-\frac{2}{2.3}-\frac{2}{3.4}-...-\frac{2}{99.100}\)
\(A=4-\left(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{99.100}\right)\)
\(A=4-2\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\right)\)
\(A=4-2\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\right)\)
\(A=4-2\left(1-\frac{1}{100}\right)\)
\(A=4-2\times\frac{99}{100}\)
\(A=4-\frac{99}{50}\)
\(A=\frac{101}{50}\)
![](https://rs.olm.vn/images/avt/0.png?1311)