2/(1*3)=1-1/3
4/3*7=(1/3-1/7)
5/7*12=1/7-1/12
7/12*19=1/12-1/19
atunci a=1-1/3+1/3-1/7+1/7-1/12+1/12-1/19=1-1/19=18/19
1/3<18/19<19/20 ?
1/3<18/19?
1/3<7/21<7/19<18/19 da ,este adevarat
deci 1/3<18/19
18/19<19/20??
18*20/(19*20)<19*19/(19*20) ?
18*20<19*19?
360<361? da ,este adevarat
deci 18/19<19/20
