2S = 2/1.3+1/3.5+2/5.7+...+2/99.100
2S = 1/1-1/3+1/3-1/5+....+1/99-1/100
2S = 1-1/100
2S = 99/100
S = 99/100:2
S = 99/200
ủng hộ mk nhé
= \(\frac{1}{2}x\left(\frac{1}{3}-\frac{1}{3}+\frac{1}{5}-\frac{1}{5}+\frac{1}{7}-\frac{1}{7}+...+\frac{1}{99}-\frac{1}{99}+\frac{1}{100}\right)\)
=\(\frac{1}{2}x\frac{1}{100}=\frac{1}{200}\)
vậy S = \(\frac{1}{200}\)