Question

[Phan Tuấn Thiện]

A=1/1x2+1/3x4+1/5x6+...+1/49x50

Chứng minh rằng A<1

Answer 1

[Akai Haruma]

Lời giải:

$A=\frac{1}{1.2}+\frac{1}{3.4}+\frac{1}{5.6}+...+\frac{1}{49.50}$

$< \frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{25.26}$

$=\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+...+\frac{26-25}{25.26}$

$=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+....+\frac{1}{25}-\frac{1}{26}$

$=1-\frac{1}{26}< 1$ (đpcm)