Question

Tính:     E=2/15 + 2/35 + 2/63 + . ..+. 2/399

Answer 1

`E=2/15+2/35+2/63+...+2/399` `E=2/(3*5)+2/(5*7)+2/(7*9)+...+2/(19*21)` `E=1/3-1/5+1/5-1/7+1/7-1/9+...+1/19-1/21` `E=1/3-1/21`

`E=2/7`

Answer 2

cái này dễ mà bạn

E=\(\dfrac{2}{15}+\dfrac{2}{35}+\dfrac{2}{63}+....+\dfrac{2}{399}\)

\(E=\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+....+\dfrac{2}{19.21}\)

E=\(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{19}-\dfrac{1}{21}\)

\(E=\dfrac{1}{3}-\dfrac{1}{21}\)

E=\(\dfrac{7}{21}-\dfrac{1}{21}\) (cái này ko ghi cũng dc nhé)

\(E=\dfrac{6}{21}=\dfrac{2}{7}\)