Question

cho S = 1/2² + 1/3² + 1/4² + ... + 1/9².

CMR: 2/5 < S < 8/9

Answer 1

ta co: 1/2^2+1/3^2+.......+1/9^2

         =1/2.2+1/3.3+.........+1/9.9

         <1/1.2+1/2.3+..........+1/8.9

         =1/1-1/2+1/2-1/3+........+1/8-1/9

         =1-1/9=8/9

=>S<8/9

a co: 1/2^2+1/3^2+.......+1/9^2

         =1/2.2+1/3.3+.........+1/9.9

         >1/2.3+1/3.4+..........+1/9.10

         =1/2-1/3+1/3-1/4+........+1/9-1/10

         =1/2-1/10=2/5

Vay S>2/5

Answer 2

like cho minh nhe. Mik làm hết sức có thể rồi đấy

Answer 3

ban nguyen quang huy lam dung roi