Bài 1:
a.x- 20/11×13 -20/13×15-20/15×17…..-20/53= 3/11
b.1/21+ 1/28+1/36+…..+2/x(x+1)=2/9
Bài 2
A=1/1x2x3+ 1/2x3x4+1/3x4x5+1/18x19x20 <1/4
B=36/1x3x5+36/3x5x7+......36/25x27x29 <3
Bài 1: a.x- 20/11×13 -20/13×15-20/15×17…..-20/53= 3/11 b.1/21+ 1/28+1/36+…..+2/x(x+1)=2/9 Bài 2 A=1/1x2x3+ 1/2x3x4+1/3x4x5+1/18x19x
By Amara
Đáp án:a.x- 20/11×13 -20/13×15-20/15×17…..-20/53
= 3/11 1/21+ 1/28+1/36+…..+2/x(x+1)
=2/9 1/1x2x3+ 1/2x3x4+1/3x4x5+1/18x19x20 <1/4 B
=36/1x3x5+36/3x5x7+……36/25x27x29
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