cho S=1/31+1/32+1/33+...+1/60 Cmr S<4/5
Cho S= 1/31 + 1/32 + 1/33 +....+ 1/59 + 1/60. CMR 3/5<S<4/5
S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)
Mà : (1/31+1/32+1/33+...+1/40) > 1/40 x 10 = 1/4 (gồm 10 số hạng)
Tương tự : (1/41 + 1/42 + ...+ 1/50) > 1/5 ; (1/51 + 1/52+...+1/59+1/60) > 1/6
S > 1/4 + 1/5 + 1/6.
Trong khi đó (1/4 + 1/5 + 1/6) > 3/5
=>S > 3/5 (1)
S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)
Mà : (1/31+1/32+1/33+...+1/40) < 1/31 x 10 = 10/30 = 1/3 (gồm 10 số hạng)
=> S < 4/5 (2)
Từ (1) và (2) => 3/5 <S<4/5
Cho S = 1/31+1/32+1/33+..............+1/60. CMR: 3/5 < S < 4/5
Cho S=1/31+1/32+1/33+.........+1/60. CMR:3/5<S<4/5
cho S = 1/31+1/32+1/33+...+1/59+1/60. cmr 3/4<S<4/5
Cho S= 1/31+1/32+1/33+...+1/60
CMR:3/5<S<4/5
Cho S=1/31+1/32+1/33+...+1/60
CMR:3/5<S<4/5
S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)
Mà : (1/31+1/32+1/33+...+1/40) > 1/40 x 10 = 1/4 (gồm 10 số hạng)
Tương tự : (1/41 + 1/42 + ...+ 1/50) > 1/5 ; (1/51 + 1/52+...+1/59+1/60) > 1/6
S > 1/4 + 1/5 + 1/6.
Trong khi đó (1/4 + 1/5 + 1/6) > 3/5
=>S > 3/5 (1)
S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)
Mà : (1/31+1/32+1/33+...+1/40) < 1/31 x 10 = 10/30 = 1/3 (gồm 10 số hạng)
=> S < 4/5 (2)
Từ (1) và (2) => 3/5 <S<4/5
cho s=1/31+1/32+1/33+..+1/60
cmr:3/5<s<4/5
Cho tổng S=1/31 +1/32 +1/33 +....+ 1/60 chứng tỏ s< 4/5
Giải:
S=\(\dfrac{1}{31}+\dfrac{1}{32}+\dfrac{1}{33}+...+\dfrac{1}{60}\)
Có 30 phân số; chia làm 3 nhóm
S<\(\left(\dfrac{1}{30}+...+\dfrac{1}{30}\right)+\left(\dfrac{1}{40}+...+\dfrac{1}{40}\right)+\left(\dfrac{1}{50}+...+\dfrac{1}{50}\right)\)
S<\(\dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}\)
S<\(\dfrac{47}{60}< \dfrac{48}{60}=\dfrac{4}{5}\)
⇒S<\(\dfrac{4}{5}\) (đpcm)
Chúc bạn học tốt!
Cho S = 1/31+1/32+1/33+.......+1/60.Chứng minh 3/5 < S < 4/
S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)
Mà : (1/31+1/32+1/33+...+1/40) > 1/40 x 10 = 1/4 (gồm 10 số hạng)
Tương tự : (1/41 + 1/42 + ...+ 1/50) > 1/5 ; (1/51 + 1/52+...+1/59+1/60) > 1/6
S > 1/4 + 1/5 + 1/6.
Trong khi đó (1/4 + 1/5 + 1/6) > 3/5
=>S > 3/5 (1)
S = (1/31+1/32+1/33+...+1/40) + (1/41 + 1/42 + ...+ 1/50) + (1/51 + 1/52+...+1/59+1/60)
Mà : (1/31+1/32+1/33+...+1/40) < 1/31 x 10 = 10/30 = 1/3 (gồm 10 số hạng)
=> S < 4/5 (2)
Từ (1) và (2) => 3/5 <S<4/5 Chúc bạn học tốt !