C=3/(12+22 )+ 11/(22+32)+....+19799/(992+1002)
Suy ra C có 99 số .
C=(1-2/5)+(1-2/13)+...+(1-2/19801)
C=99-(2/5+2/13+...+2/19801)
bây giờ chúng ta phải chứng minh biểu thức trong ngoặc nhỏ hơn 1.
Xét B=2/5+2/13+...+2/19801
B<2/4+2/12+...+2/19800
B<1/2+1/6+...+1/9900
B<1/1x2+1/2x3+...+1/99x100
B<1-1/100<1
Vậy B < 1 thì C> 99-1=98
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