Chứng minh rằng: N = 1/4^2 + 1/6^2 + 1/8^2 + ... + 1/(2n)^2 < 1/4

2.Chung minh rang:

a) N=1/4^2+1/6^2+1/8^2+...+1/(2n)^2 < 1/4

b) 1/26+1/27+1/28+...+1/50=1-1/2+1/3-1/4+...+1/49-1/50

c) A=1/6+1/24+1/60+...+1/6840 < 1/4

d) B= 36/15+36/105+36/315+...+36/19575 < 3