1/31>1/40

1/32>1/40

...

1/40=1/40

=>1/31+1/32+...+1/40>1/40*10=1/4

1/41>1/50

1/42>1/50

...

1/50=1/50

=>1/41+1/42+...+1/50>10/50=1/5

1/51>1/60

1/52>1/60

...

1/60=1/60

=>1/51+1/52+...+1/60>10/60=1/6

=>S>1/4+1/5+1/6=3/5

1/31<1/30

1/32<1/30

...

1/40<1/30

=>1/31+1/32+...+1/40<1/30*10=1/3

1/41<1/40

1/42<1/40

...

1/50<1/40

=>1/41+1/42+...+1/50<10/40=1/4

1/51<1/50

1/52<1/50

...

1/60<1/50

=>1/51+1/52+...+1/60<10/50=1/5

=>S<1/3+1/4+1/5=4/5

A = (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

A > 1/4 + 1/5 + 1/6.

Trong khi đó (1/4 + 1/5 + 1/6) > 3/5

Vậy A > 3/5 (1)

Mặt khác

A = (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)

Tương tự : (1/41 + 1/42 + ...+ 1/50)  < 1/4 ;   (1/51 + 1/52+...+1/59+1/60) < 1/5

Mà A = (1/3 + 1/4 + 1/5) < 4/5 (Vì 1/3 + 1/5 < 3/5 hay 7/12 < 3/5 hay 35/60 < 36/60)

Vậy A <  4/5 (2)

Từ (1);(2)=> 3/5 <S <4/5 (dpcm)

Ta có:

S=131+132+133+...+160S=131+132+133+...+160

⇒S=(131+132+...+140)+(141+142+...+150)+(151+152+...+160)⇒S=(131+132+...+140)+(141+142+...+150)+(151+152+...+160)

Nhận xét:

131+132+...+140>140+140+...+140=14131+132+...+140>140+140+...+140=14

141+142+...+150>150+150+...+150=15141+142+...+150>150+150+...+150=15

151+152+...+160>160+160+...+160=16151+152+...+160>160+160+...+160=16

⇒S>14+15+16=3760>35⇒S>14+15+16=3760>35

⇒S>35(1)⇒S>35(1)

Lại có:

S=(131+132+...+140)+(141+142+...+150)+(151+152+...+160)S=(131+132+...+140)+(141+142+...+150)+(151+152+...+160)

Nhận xét:

131+132+...+140<130+130+...+130=13131+132+...+140<130+130+...+130=13

141+142+...+150<140+140+...+140=14141+142+...+150<140+140+...+140=14

151+152+...+160<150+150+...+150=15151+152+...+160<150+150+...+150=15

⇒S<13+14+15=4760<45⇒S<13+14+15=4760<45

⇒S<45(2)⇒S<45(2)

Từ (1)(1) và (2)(2)

⇒35<S<45⇒35<S<45 (Đpcm)

mình nhằm nha

để gửi lại ,

xin lỗi nhiều

hì

S sẽ có 30 số hạng. Nhóm thành 3 nhóm, mỗi nhóm 101 số hạng.

S= (1/31+1/32+...+1/40) + (1/41 + 1/42 +...+1/50) + (1/51 +1/52+...+1/60)

S < (1/30 + 1/30 +...+ 1/30) + ( 1/40 +1/40+...+1/40) + (1/50 +1/50+...+1/50)

S < 1/30 + 1/40 +1/50 ; S < 47/60 < 48/60 = 4/5 (1)

S > (1/40 + 1/40 +...=1/40) + (1/50 + 1/50 +...+1/50) + (1/60 +1/60+...+1/60)

S < 10/40 + 10/50 +10/60 ; S > 37/60 > 36/60 = 3/5 (2)

Tư (1) và (2) => 3/5 < S < 4/5

\(\dfrac{1}{31}>\dfrac{1}{40}\)

\(\dfrac{1}{32}>\dfrac{1}{40}\)

...

\(\dfrac{1}{40}=\dfrac{1}{40}\)

=>\(\dfrac{1}{31}+\dfrac{1}{32}+...+\dfrac{1}{40}>\dfrac{1}{40}+\dfrac{1}{40}+...+\dfrac{1}{40}=\dfrac{10}{40}=\dfrac{1}{4}\)

\(\dfrac{1}{41}>\dfrac{1}{50}\)

\(\dfrac{1}{42}>\dfrac{1}{50}\)

...

\(\dfrac{1}{50}=\dfrac{1}{50}\)

=>\(\dfrac{1}{41}+\dfrac{1}{42}+...+\dfrac{1}{50}>\dfrac{1}{50}+\dfrac{1}{50}+...+\dfrac{1}{50}=\dfrac{10}{50}=\dfrac{1}{5}\)

\(\dfrac{1}{51}>\dfrac{1}{60}\)

\(\dfrac{1}{52}>\dfrac{1}{60}\)

...

\(\dfrac{1}{60}=\dfrac{1}{60}\)

=>\(\dfrac{1}{51}+\dfrac{1}{52}+...+\dfrac{1}{60}>\dfrac{1}{60}+\dfrac{1}{60}+...+\dfrac{1}{60}=\dfrac{10}{60}=\dfrac{1}{6}\)

=>\(S>\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}=\dfrac{3}{5}\)

\(\dfrac{1}{31}< \dfrac{1}{30}\)

\(\dfrac{1}{32}< \dfrac{1}{30}\)

...

\(\dfrac{1}{40}< \dfrac{1}{30}\)

=>\(\dfrac{1}{31}+\dfrac{1}{32}+...+\dfrac{1}{40}< \dfrac{1}{30}+\dfrac{1}{30}+...+\dfrac{1}{30}=\dfrac{10}{30}=\dfrac{1}{3}\)

\(\dfrac{1}{41}< \dfrac{1}{40}\)

\(\dfrac{1}{42}< \dfrac{1}{40}\)

...

\(\dfrac{1}{50}< \dfrac{1}{40}\)

=>\(\dfrac{1}{41}+\dfrac{1}{42}+...+\dfrac{1}{50}< \dfrac{1}{40}+\dfrac{1}{40}+...+\dfrac{1}{40}=\dfrac{10}{40}=\dfrac{1}{4}\)

\(\dfrac{1}{51}< \dfrac{1}{50}\)

\(\dfrac{1}{52}< \dfrac{1}{50}\)

...

\(\dfrac{1}{60}< \dfrac{1}{50}\)

=>\(\dfrac{1}{51}+\dfrac{1}{52}+...+\dfrac{1}{60}< \dfrac{1}{50}+\dfrac{1}{50}+...+\dfrac{1}{50}=\dfrac{10}{50}=\dfrac{1}{5}\)

=>\(S< \dfrac{1}{3}+\dfrac{1}{4}+\dfrac{1}{5}=\dfrac{4}{5}\)

=>\(\dfrac{3}{5}< S< \dfrac{4}{5}\)

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 Chúc bạn học tốt !

Tham khảo:

S có 30 số hạng . Nhóm thành 3 nhóm , mỗi nhóm có 10 số hạng 

S = (1/31+1/32+....+1/40)+(1/41+1/42+....+1/50)+(1/51+1/52+....+1/60)

< (1/30+1/30+.....+1/30)+(1/40+1/40+......+1/40)+(1/50+1/50+....+1/50)

= 10/30 + 10/40 + 10/50 = 47/60 < 48/60 = 4/5 (1)

Lại có : S > (1/40+1/40+.....+1/40)+(1/50+1/50+....+1/50)+(1/60+1/60+.....+1/60)

= 10/40 + 10/50 + 10/60 = 37/60 > 36/60 = 3/5 (2)

Từ (1) và (2) => 3/5 < S < 4/5

=> ĐPCM

Tk mk nha

\(S=\frac{1}{31}+\frac{1}{32}+..+\frac{1}{60}< \left(\frac{1}{31}+...+\frac{1}{40}\right)+\left(\frac{1}{41}+...+\frac{1}{50}\right)+\left(\frac{1}{51}+..+\frac{1}{60}\right)< \frac{1}{3}+\frac{1}{4}+\frac{1}{5}=\frac{47}{60}< \frac{48}{60}=\frac{4}{5}\)

\(S=\frac{1}{31}+\frac{1}{32}+..+\frac{1}{60}>\left(\frac{1}{31}+...+\frac{1}{40}\right)+\left(\frac{1}{41}+...+\frac{1}{50}\right)+\left(\frac{1}{51}+..+\frac{1}{60}\right)>\frac{1}{4}+\frac{1}{5}+\frac{1}{6}=\frac{37}{60}>\frac{36}{60}=\frac{3}{5}\)

Vậy...