1/1.3 + 1/3.5 +....+1/17.19=1/2( 2/1.3+2/3.5+2/5.7+....+2/17.19)
= 1/2 ( 1-1/3+1/3-1/5+...........+1/17-1/19)
= 1/2( 1-1/19)
=9/19
bạn có thể tham khảo cái này nhé:1/m-1/n=n-m/m.n
B=\(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{17.19}\)
\(2B=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{17.19}\)
\(2B=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{17}-\frac{1}{19}\)
\(2B=1-\frac{1}{19}=\frac{18}{19}\)
\(B=\frac{18}{19}.\frac{1}{2}=\frac{9}{19}\)
k mk nha
