Question

`A=1/1.6+1/6.11+1/11.16+1/16.21+...+1/31.36`

`B=1/3.9+1/9.15+1/15.21+1/21.27+...+1/39.45`

Áp dụng công thức : `1/ (n.(n+m) )` `=1/nxx( 1/n-1/(n+m) )`

Answer 1

cíu:")

Answer 2

hm xu mới hỏi :>

Answer 3

`#seenkhongrep?`

`A=1/1.6+1/6.11+1/11.16+1/16.21+...+1/31.36`

`5A=5/1.6+5/6.11+5/11.16+...+5/31.36`

`5A=1-1/6+1/6-1/11+1/11-1/16+...+1/31-1/36`

`5A=1-1/36`

`5A=36/36-1/36`

`5A=35/36`

`A=35/36:5`

`A=7/36`

Vậy `A=7/36.`

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`B=1/3.9+1/9.15+1/15.21+...+1/39.45`

`6B=6/3.9+6/9.15+6/15.21+...+6/39.45`

`6B=1/3−1/9+1/9−1/15+1/15−1/21+...+1/39−1/45`

`6B=1/3−1/45`

`6B=14/45`

`B=14/45:6`

`B=71/35`