Question

Tính: 2/15 + 2/35 + 2/63+.....+ 2/399

 

Answer 1

bài này dễ mà

Answer 2

\(A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\) 

\(A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)

\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\)

\(A=\frac{1}{3}-\frac{1}{21}=\frac{2}{7}\)

Answer 3

2/15 + 2/35 + 2/63 + ... + 2/399

= 2/3×5 + 2/5×7 + 2/7×9 + ... + 2/19×21

= 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/19 - 1/21

= 1/3 - 1/21

= 7/21 - 1/21

= 6/21 = 2/7