Chứng tỏ rằng : 1/3.4 + 1/4.5 +...+ 1/19/20 < 1/2
Giải thích
a)13.4+14.5+...+119.20=13−120=1760<12
b)31.4+34.7+37.10+...+397.100=1−1100<1
c)Ta có :122+132+142+...+192>12.3+13.4+14.5+...+19.10=25
122+132+142+...+192<11.2+12.3+13.4+18.9=89
a)13.4+14.5+...+119.20=13−120=1760<12
b)31.4+34.7+37.10+...+397.100=1−1100<1
c)Ta có :122+132+142+...+192>12.3+13.4+14.5+...+19.10=25
122+132+142+...+192<11.2+12.3+13.4+18.9=89