Gọi A = 1/1 . 2^2 + 1/2 . 3^2 + 1/3 . 4^2 + ... + 1/49 . 50^2

Gọi A= 1/1.2^2+1/2.3^2+1/3.4^2+...+1/49.50^2

B=1/2^2+1/3^2+...+1/50^2

CMR A<1/2<B