Đặt :
\(A=\dfrac{2}{11.15}+\dfrac{2}{15.19}+.......+\dfrac{2}{51.55}\)
\(\Leftrightarrow2A=\dfrac{4}{11.15}+\dfrac{4}{15.19}+.........+\dfrac{4}{51.55}\)
\(\Leftrightarrow2A=\dfrac{1}{11}-\dfrac{1}{15}+\dfrac{1}{15}-\dfrac{1}{19}+......+\dfrac{1}{51}-\dfrac{1}{55}\)
\(\Leftrightarrow2A=\dfrac{1}{11}-\dfrac{1}{55}\)
\(\Leftrightarrow2A=\dfrac{4}{55}\)
\(\Leftrightarrow A=\dfrac{4}{110}\)
Đúng 1
Bình luận (0)
