Chứng tỏ rằng : 2/5 < 1/2^2 + 1/3^2 + 1/4^2 + 1/9^2 < 8/9
Giải thích
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
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