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3 tháng 5 2015

\(S=\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}=\frac{7-\left(b+c\right)}{b+c}+\frac{7-\left(c+a\right)}{c+a}+\frac{7-\left(a+b\right)}{a+b}\)

                                               \(=\frac{7}{b+c}-\frac{b+c}{b+c}+\frac{7}{c+a}-\frac{c+a}{c+a}+\frac{7}{a+b}-\frac{a+b}{a+b}\)

                                                \(=\frac{7}{b+c}-1+\frac{7}{c+a}-1+\frac{7}{a+b}-1\)

                                                \(=\frac{7}{b+c}+\frac{7}{c+a}+\frac{7}{a+b}-3\)  

                                                \(=7.\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)-3\) \(.Thay\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}=\frac{7}{10}\)

                                               \(\Rightarrow S=7.\frac{7}{10}-3=\frac{49}{10}-3=1\frac{9}{10}>1\frac{8}{11}\)

                                              Vậy\(S>1\frac{8}{11}\)

24 tháng 2 2017

S>19/11

100% luôn

Bài 1 :

\(A=\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+...+\frac{50-49}{49.50}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{3}+...+\frac{1}{49}-\frac{1}{50}\)

\(=1-\frac{1}{50}< 1\left(1\right)\)

\(B=\frac{1}{10}+\left(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}\right)\)\(>\frac{1}{10}+\frac{1}{100}.90=1\left(2\right)\)

Từ (1) và ( 2) ta có \(A< 1\) \(B>1\)NÊN \(A< B\)

Bài 2:

\(S=\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\)

\(=\frac{\left(a+b+c\right)-\left(b+c\right)}{b+c}+\)\(\frac{\left(a+b+c\right)-\left(c+a\right)}{c+a}\)\(+\frac{\left(a+b+c\right)-\left(a+b\right)}{a+b}\)

\(=\frac{7-\left(b+c\right)}{b+c}+\frac{7-\left(c+a\right)}{c+a}+\frac{7-\left(a+b\right)}{a+b}\)

\(=7.\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)-3\)

\(=7.\frac{7}{10}-3\)\(=\frac{49}{10}-3=\frac{19}{10}\)

\(S=\frac{19}{10}>\frac{19}{11}=1\frac{8}{11}\)

Chúc bạn học tốt ( -_- )

2 tháng 6 2018

Bài 1:

ta có: \(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{49.50}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{49}-\frac{1}{50}\)

\(A=1-\frac{1}{50}< 1\)

\(\Rightarrow A< 1\)(1) 

ta có: \(\frac{1}{11}>\frac{1}{100};\frac{1}{12}>\frac{1}{100};...;\frac{1}{99}>\frac{1}{100}\)

\(\Rightarrow\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}>\frac{1}{100}+\frac{1}{100}+...+\frac{1}{100}+\frac{1}{100}\) ( có 90 số 1/100)

                                                                               \(=\frac{90}{100}=\frac{9}{10}\)

\(\Rightarrow B=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+...+\frac{1}{99}+\frac{1}{100}>\frac{1}{10}+\frac{9}{10}=1\)

\(\Rightarrow B>1\)(2)

Từ (1);(2) => A<B

24 tháng 2 2019

Xét

 \(\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}\right)=7\cdot\frac{7}{10}=\frac{49}{10}\)

\(\Leftrightarrow\frac{a+b}{a+b}+\frac{c}{a+b}+\frac{a+c}{a+c}+\frac{b}{a+c}+\frac{b+c}{b+c}+\frac{a}{b+c}=\frac{49}{10}\)

\(3+\frac{a}{b+c}+\frac{b}{a+c}+\frac{c}{a+b}=\frac{49}{10}\Leftrightarrow S=\frac{19}{10}\)

Ta có:   \(1\frac{8}{11}=\frac{19}{11}\)

vì 19=19 ,\(\frac{1}{11}< \frac{1}{10}\)nên \(\frac{19}{11}< \frac{19}{10}\)

Vậy \(S>1\frac{8}{11}\)

4 tháng 5 2019

\(A=\frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}\)

    \(=\frac{a}{b+c}+1+\frac{b}{c+a}+1+\frac{c}{a+b}+1-3\)

    \(=\frac{a+b+c}{b+c}+\frac{a+b+c}{c+a}+\frac{a+b+c}{a+b}-3\)

    \(=\left(a+b+c\right)\left(\frac{1}{b+c}+\frac{1}{c+a}+\frac{1}{a+b}\right)-3\)

     \(=7.\frac{7}{10}-3=\frac{49}{10}-3=\frac{19}{10}\)

Ta có:\(1\frac{8}{11}=\frac{19}{11}< \frac{19}{10}\left(đpcm\right)\)

V...

NV
2 tháng 5 2019

\(A=\frac{a}{b+c}+1+\frac{b}{a+c}+1+\frac{c}{a+b}+1-3\)

\(A=\frac{a+b+c}{b+c}+\frac{a+b+c}{a+c}+\frac{a+b+c}{a+b}-3\)

\(A=\left(a+b+c\right)\left(\frac{1}{a+b}+\frac{1}{a+c}+\frac{1}{b+c}\right)-3\)

\(A=7.\frac{7}{10}-3=\frac{49}{10}-3=\frac{19}{10}>\frac{19}{11}=1\frac{8}{11}\)

Đề sai

20 tháng 2 2017

a) Ta có :

\(A=\frac{\frac{2}{5}-\frac{2}{9}+\frac{2}{11}}{\frac{7}{5}-\frac{7}{9}+\frac{7}{11}}\)

\(A=\frac{2\left(\frac{1}{5}-\frac{1}{9}+\frac{1}{11}\right)}{7\left(\frac{1}{5}-\frac{1}{9}+\frac{1}{11}\right)}\)

\(A=\frac{2}{7}\)

b) Ta có :

\(B=\frac{\frac{1}{3}-\frac{1}{4}+\frac{1}{5}}{\frac{7}{6}-\frac{7}{8}+\frac{7}{10}}\)

\(B=\frac{\frac{1}{3}-\frac{1}{4}+\frac{1}{5}}{\frac{7}{2}\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{5}\right)}\)

\(B=\frac{2}{7}\)

\(\Rightarrow\frac{A}{B}=\frac{\frac{2}{7}}{\frac{2}{7}}=1\)

29 tháng 4 2016

\(A=1\)

\(B=\)

29 tháng 4 2016

\(A=\frac{11}{9}-\frac{7}{8}+-\frac{2}{3}-\frac{1}{8}+\frac{25}{9}-\frac{4}{3}\)

\(A=1\)

\(B=1\frac{3}{4}:\frac{3}{5}-\frac{2}{3}x1,75+\left(\frac{1}{2}\right)^2:\frac{1}{7}\)

\(B=3,5\)