\(S=\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+....+\frac{1}{20}\)

\(=\left(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}\right)+\left(\frac{1}{16}+\frac{1}{17}+\frac{1}{18}+\frac{1}{19}+\frac{1}{20}\right)\)

\(>\frac{1}{15}\cdot5+\frac{1}{20}\cdot5\)

\(=\frac{1}{3}+\frac{1}{4}\)

\(=\frac{7}{12}>\frac{6}{12}=\frac{1}{2}\)

\(\Rightarrow S>\frac{1}{2}\)

Bài làm

Ta có: 

\(\frac{1}{11}>\frac{1}{20}\), \(\frac{1}{12}>\frac{1}{20}\), \(\frac{1}{13}>\frac{1}{20}\), \(\frac{1}{14}>\frac{1}{20}\), \(\frac{1}{15}>\frac{1}{20}\), \(\frac{1}{16}>\frac{1}{20}\), \(\frac{1}{17}>\frac{1}{20}\), \(\frac{1}{18}>\frac{1}{20}\),\(\frac{1}{19}>\frac{1}{20}\)

=> \(S=\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}+\frac{1}{16}+\frac{1}{17}+\frac{1}{18}+\frac{1}{19}+\frac{1}{20}>\frac{1}{20}\)

hay \(\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}+\frac{1}{20}\)

=> \(S=\frac{1}{20}.10=\frac{10}{20}=\frac{1}{2}\)

Do đó: \(S=\frac{1}{2}\)

# Chúc bạn học tốt #

Ta có các phân số : \(\frac{1}{11};\frac{1}{12};\frac{1}{13};\frac{1}{14};\frac{1}{15};\frac{1}{16};\frac{1}{17};\frac{1}{18};\frac{1}{19}>\frac{1}{20}\)

Do đó : \(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}+\frac{1}{16}+\frac{1}{17}+\frac{1}{18}+\frac{1}{19}+\frac{1}{20}>\frac{1}{20}+\frac{1}{20}+...+\frac{1}{20}\)có 10 phân số \(\frac{1}{20}\)

\(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}+\frac{1}{16}+\frac{1}{17}+\frac{1}{18}+\frac{1}{19}+\frac{1}{20}>\frac{10}{20}\)

\(\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\frac{1}{15}+\frac{1}{16}+\frac{1}{17}+\frac{1}{18}+\frac{1}{19}+\frac{1}{20}>\frac{1}{2}\)

Vậy : \(S>\frac{1}{2}\)

\(31^{11}< 32^{11}=\left(2^5\right)^{11}=2^{55}\)

\(17^{14}>16^{14}=\left(2^4\right)^{14}=2^{56}\)

\(\Rightarrow17^{14}>2^{56}>2^{55}>31^{11}\)

\(31^{11}<32^{11}=\left(2^5\right)^{11}=16^{11}<16^{14}<17^{14}\)

Vậy \(31^{11}<17^{14}\).

31^11  >  17^14

\(31^{11}<32^{11}=\left(2^5\right)^{11}=2^{55};17^{14}>16^{14}=\left(2^4\right)^{14}=2^{56}\)

Vì 2⁵⁵ < 2⁵⁶ nên 31¹¹ < 17¹⁴.

31¹¹ và 17¹⁴

31¹¹<32¹¹=(2⁵)¹¹=2⁵⁵

=>31¹¹<2⁵⁵

17¹⁴>16¹⁴=(2⁴)¹⁴=2⁵⁶

=>17¹⁴>2⁵⁶

=>31¹¹<2⁵⁵<2⁵⁶<17¹⁴

=>31¹¹<17¹⁴

\(31^{11}< 32^{11}=\left(2^5\right)^{11}=2^{55}< 2^{56}=\left(2^4\right)^{14}=16^{14}< 17^{14}\)

\(=>31^{11}< 17^{14}\)

Vậy........

ví dụ 16, trang 13, sách toán bồi dưỡng học sinh 6, bạn mở ra mà xem :))

Ta có:

31¹¹ < 32¹¹ = (2⁵)¹¹ = 2⁵⁵

17¹⁴ > 16¹⁴ = (2⁴)¹⁴ = 2⁵⁶

Vì 2⁵⁵ < 2⁵⁶ => 32¹¹ < 16¹⁴ => 31¹¹ < 17¹⁴

Ta có : \(31^{11}< 32^{11}=\left(2^5\right)^{11}=2^{55}\)

\(\Rightarrow31^{11}< 2^{55}\)

\(17^{14}>16^{14}=\left(2^4\right)^{14}=2^{56}\)

\(\Rightarrow17^{14}>2^{56}\)

\(\Rightarrow31^{11}< 2^{55}< 2^{56}< 17^{14}\)

\(\Rightarrow31^{11}< 17^{14}\)

BẠN LÀM THẾ SAI RỒI
LÀM NHƯ VẬY VÍ DỤ: 5>3: 6>2 =>> 5>6

Vì (-17)¹⁴<0(Vì có -17)

    31¹¹>0(vì hai số 31;11 là số nguyên dương)

             Suy ra:31¹¹>(-17)¹⁴

\(31^{11}< 32^{11}=\left(2^5\right)^{11}=2^{55}\)

\(2^{55}< 2^{56}=\left(2^4\right)^{14}=16^{14}< \left(-17\right)^{14}\)

=> \(31^{11}< \left(-17\right)^{14}\)