Question

Không quy đồng hãy tính tổng sau: .A= -1/ 20+ -1/30+ -1/42+ -1/56+ -1/72+ -1/90

Answer 1

\(A=\dfrac{-1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\)

\(A=\dfrac{-1}{4\cdot5}+\dfrac{-1}{5\cdot6}+\dfrac{-1}{6\cdot7}+\dfrac{-1}{7\cdot8}+\dfrac{-1}{8\cdot9}+\dfrac{-1}{9\cdot10}\)

\(A=\dfrac{-1}{4}-\dfrac{-1}{5}+\dfrac{-1}{5}-\dfrac{-1}{6}+\dfrac{-1}{6}-\dfrac{-1}{7}+\dfrac{-1}{7}-\dfrac{-1}{8}+\dfrac{-1}{8}-\dfrac{-1}{9}+\dfrac{-1}{9}-\dfrac{-1}{10}\)

\(A=\dfrac{-1}{4}-\dfrac{-1}{10}\)

\(A=-\dfrac{3}{20}\)

Answer 2

Answer 3

Answer 4

\(A=\dfrac{-1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\)

\(A=\dfrac{-1}{4.5}+\dfrac{-1}{5.6}+\dfrac{-1}{6.7}+\dfrac{-1}{7.8}+\dfrac{-1}{8.9}+\dfrac{-1}{9.10}\)

\(A=\dfrac{-1}{4}-\dfrac{-1}{5}+\dfrac{-1}{5}-\dfrac{-1}{6}+\dfrac{-1}{6}-\dfrac{-1}{7}+\dfrac{-1}{7}-\dfrac{-1}{8}+\dfrac{-1}{8}-\dfrac{-1}{9}+\dfrac{-1}{9}-\dfrac{-1}{10}\)

\(A=\dfrac{-1}{4}-\left(\dfrac{1}{5}+\dfrac{-1}{5}+\dfrac{1}{6}+\dfrac{1}{6}+\dfrac{1}{7}+\dfrac{-1}{7}+\dfrac{1}{8}+\dfrac{-1}{8}+\dfrac{1}{9}+\dfrac{-1}{9}\right)-\dfrac{-1}{10}\)

\(A=\dfrac{-1}{4}-\dfrac{-1}{10}\)

\(A=\dfrac{-1}{4}+\dfrac{1}{10}=\dfrac{-3}{20}\)