Question

5²\1.6+5²\6.11+...+5²\26.31

    nhanh lên nhé e sắp phải nộp rồi

      \:phần     .:dau nhan

Answer 1

\(\text{Đặt }A=\frac{5^2}{1.6}+\frac{5^2}{6.11}+...+\frac{5^2}{26.31}\)

\(=5.\left(\frac{5}{1.6}+\frac{5}{6.11}+...+\frac{5}{26.31}\right)\)

\(=5.\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{26}-\frac{1}{31}\right)\)

\(=5.\left(1-\frac{1}{31}\right)\)

\(=5.\frac{30}{31}=\frac{150}{31}\)

Vậy A = 150/31.