a) S = 1/5 + ( 1/13 + 1/14 + 1/15 ) + ( 1/61 + 1/62 + 1/63)
< 1/5 + 3. 1/12 + 3. 1/60 = 1/5 + 1/4 + 1/20 = 1/2
b) Ta có: 1/2.2 < 1/1.2
........
1/50.50 < 1/49.50
=>M<1-1/2+1/2-1/3+....+1/49-1/50 = 1 - 1/50 <1 =>M <1
Mà 173/100 > 1 => M < 173/100
c) N = 1 - 1/4 + 1/4 - 1/7 + ... + 1/40 - 1/43 = 1 - 1/43 < 1