a)2/3x5+2/5x7+...+2/97x99
=1/3-1/5+1/5-1/7+...+1/97-1/99
=1/3-1/99
=32/99
Mà 32/99>32%=>2/3x5+2/5x7+...+2/97x99>32%
b)1/21+1/22+...+1/40>7/12
Ta có 1/21<1/30;1/22>1/30;...;1/29>1/30
=>1/21+1/22+...+1/30>1/30+...+1/30=1/3 (1)
1/31>1/40;1/32>1/40;...;1/39>1/40
=>1/31+1/32+...+1/39+1/40>1/40+...+1/40=1/4 (2)
Từ (1) và (2)=>1/21+1/22+...+1/30+1/31+1/32+...+1/39+1/40>1/3+1/4
=>1/21+1/22+...+1/40>7/12