Question

S = 1/4 + 1/9 + 1/16 + 1/25 + 1/36 + 1/49 + 1/64 + 1/81

CMR: 2/2 < S < 8/9

Answer 1

S=1/4+1/9+1/16+1/25+1/36+1/49+1/64+1/81=1-1/81=1/81

 

Answer 2

80/81 là đúng

Answer 3

vô lí vì 2/2 = 1 mà 8/9 < 1