Question

Tính 

\(B=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}\)

Answer 1

B=2/15+2/35+2/63+2/99

B=2(1/3.5+1/5.7+1/7.9+1/9.11) khoảng cách từ 3-5;5-5;7-9;9-11 là 2 nen

B=2/2(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11) gop -1/5+1/5;-1/7+1/7;-1/9+1/9=0

B=1(1/3-1/11)=8/33

Answer 2

 =2/3*5+2/5*7+2/7*9+2/9*11

=1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11

=1/3-1/11

=8/33

Answer 3

\(B=\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+\frac{2}{9\cdot11}\)

\(B=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)

\(B=\frac{1}{3}-\frac{1}{11}\)

\(B=\frac{11}{33}-\frac{3}{33}\)

\(B=\frac{8}{33}\)