Question

giúp e vs ạ

 

Answer 1

\(A=\dfrac{1}{7}+\dfrac{1}{13}+\dfrac{1}{21}+...+\dfrac{1}{n^2+3\left(n+1\right)}\)

=>\(A< \dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{n+1}-\dfrac{1}{n+2}\)

=>A<1/2-1/(n+2)

=>A<(n+2-2)/2(n+2)=n/2(n+2)