Question

Cho S = 3/10+3/11+3/12+3/13+3/14.CMR:1<S<2

 

Answer 1

Ta có: S =3/10+3/11+3/12+3/13+3/14 = 3.(1/10+1/11+1/12+1/13+1/14) > 3.(1/15 + 1/15 + 1/15 + 1/15 + 1/15) = 3.5/15 = 1 => S > 1       (1)

         S=3/10+3/11+3/12+3/13+3/14 = 3.(1/10+1/11+1/12+1/13+1/14) < 3.(1/10 + 1/10 + 1/10 + 1/10 + 1/10) = 3.5/10 = 3/2<2 =>S <2  (2)

 Từ (1) va (2)

=> 1 < S < 2 (đpcm).

Chúc bạn học tập tốt :)