Tính tổng 6/8 + 6/56 + 6/140 + ... + 6/1100 + 6/1400.
Giải thích
68+656+6140+...+61100+61400=2.32.4+2.32.28+2.32.70+...+2.32.550+2.32.700=34+328+370+...+3550+3700=31.4+34.7+37.10+...+322.25+325.28=1−14+14−17+17−110+...+122−125+125−128=1−128=2828−128=2728