Question

[Ngoc Linh]

Chứng tỏ rằng:

1/6+1/7+1/8+1/9+.......+1/18+1/19<2

Answer 1

[Arima Kousei]

Ta có : 

1/6 < 1/5 , 1/7 < 1/5 , ... 1/19 < 1/5

=> 1/6 + 1/7 + ...+ 1/19 < 1/5 + 1/5 + ...+ 1/5

=> 1/6 + 1/7 + ...+ 1/19  < 1/5 . 14 

=> 1/6 + 1/7 + ...+ 1/19 < 14/5 = 2 , 8