Cho mình sửa lại một chút nha , làm nhầm
\(C=\dfrac{1}{5}+\dfrac{1}{35}+\dfrac{1}{63}+...+\dfrac{1}{143}\)
\(=\dfrac{1}{5}+\dfrac{1}{5}.\dfrac{1}{7}+\dfrac{1}{7}.\dfrac{1}{9}+-+\dfrac{1}{11}.\dfrac{1}{13}\)
\(=\dfrac{1}{5}+\dfrac{1}{2}\left(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{11}-\dfrac{1}{13}\right)\)
\(=\dfrac{1}{5}+\dfrac{1}{2}.\left(\dfrac{1}{5}-\dfrac{1}{13}\right)=\dfrac{1}{5}+\dfrac{1}{2}.\left(\dfrac{8}{65}\right)\)
\(=\dfrac{1}{5}+\dfrac{4}{65}\)
\(=\dfrac{17}{65}\)
Vậy C = 17/65
Chúc bn hok tốt
