Chứng minh rằng :
a) M = \(\frac{1}{2^2}\)\(+\)\(\frac{1}{3^2}\)\(+\)\(\frac{1}{4^2}\)\(+\)... \(+\)\(\frac{1}{n^2}\) < 1 ( n \(\in\)N , n \(\ge\)2 )
b) N = \(\frac{1}{4^2}\)\(+\)\(\frac{1}{6^2}\)\(+\)\(\frac{1}{8^2}\)\(+\)... \(+\)\(\frac{1}{\left(2n\right)^2}\)< \(\frac{1}{4}\)( n \(\in\)N , n \(\ge\)2 )
c) P = \(\frac{2!}{3!}\)\(+\)\(\frac{2!}{4!}\)\(+\)\(\frac{2!}{5!}\)\(+\)... \(+\)\(\frac{2!}{n!}\)< 1 ( n \(\in\)N , n \(\ge\)3 )