Question

Tính nhanh : 1/56 + 1/72 + 1/90 + 1/110
Giúp mik vs nhé . Mai mik phải nộp bài rồi

Answer 1

\(\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}+\dfrac{1}{110}\)

\(=\dfrac{1}{7\cdot8}+\dfrac{1}{8\cdot9}+\dfrac{1}{9\cdot10}+\dfrac{1}{10\cdot11}\)

\(=\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}\)

\(=\dfrac{1}{7}-\dfrac{1}{11}\)

\(=\dfrac{11}{77}-\dfrac{7}{77}\)

\(=\dfrac{4}{77}\)

Answer 2

\(=\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}+\dfrac{1}{10.11}=\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}\)

\(=\dfrac{1}{7}-\dfrac{1}{11}=\dfrac{11}{77}-\dfrac{7}{77}=\dfrac{4}{77}\)