iei ecuatia pe bucati:
demonstrezi prima data : 1/2 < 1/20+1/21+1/22+...+1/39
1/40 <1/20
1/40<1/21
...................
1/40<1/39
============(aduni)
1/40+1/40 +...+1/40 (de 20 ori ) <1/20+..+1/39
1/2=20/40 <1/20+..+1/39
acum cealalta parte :1/20+1/21+1/22+...+1/39 < 1
1/20≤1/20
1/21 <1/20
...................
1/39<1/20
============
1/20+1/21+1/22+...+1/39 < 1/20+..+1/20 (de 20 ori) =20/20=1
