Question

[Phương Quỳnh]

Tính \(M=\dfrac{-4}{1.5}-\dfrac{4}{5.9}-\dfrac{4}{9.13}-...-\dfrac{4}{\left(n+4\right)n}\)

Answer 1

[nguyển thị việt hà]

\(-\left(\dfrac{4}{1.5}+\dfrac{4}{5.9}+\dfrac{4}{9.13}...+\dfrac{4}{n\left(n+4\right)}\right)\) \(=-\left(1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+...+\dfrac{1}{n}+\dfrac{1}{n+4}\right)=-\left(1-\dfrac{1}{n+4}\right)=-1+\dfrac{1}{n+4}\)