Đề là j

để là quy đồng mẫu số

a) 10/7 - 3/8 

= 59/56

b) 9/10- 7/12

= 19/60

giúp mình với

a) \(\dfrac{5}{4}+\dfrac{3}{5}=\dfrac{25}{20}+\dfrac{12}{20}=\dfrac{37}{20}\)

b) \(\dfrac{7}{8}+\dfrac{9}{24}=\dfrac{21}{24}+\dfrac{9}{24}=\dfrac{30}{24}=\dfrac{15}{12}\)

c) \(\dfrac{2}{36}+\dfrac{1}{6}+\dfrac{5}{12}=\dfrac{2}{36}+\dfrac{6}{36}+\dfrac{15}{36}=\dfrac{23}{36}\)

\(\dfrac{1}{2}=\dfrac{6}{12};\dfrac{2}{3}=\dfrac{8}{12};\dfrac{7}{12}=\dfrac{7}{12}\)

Chắc là quy đồng nhỉ :)

\(\dfrac{1\times6}{2\times6}=\dfrac{6}{12};\dfrac{2\times4}{3\times4}=\dfrac{8}{12};\dfrac{7}{12}\)

\(\dfrac{5}{11}+\dfrac{4}{11}=\dfrac{9}{11}\\ \dfrac{11}{10}+\dfrac{9}{10}=\dfrac{20}{10}=2\\ \dfrac{11}{12}+\dfrac{5}{12}=\dfrac{16}{12}=\dfrac{4}{3}\\ \dfrac{7}{9}+\dfrac{5}{9}=\dfrac{12}{9}=\dfrac{4}{3}\)

\(\dfrac{5}{11}+\dfrac{4}{11}=1\)

\(\dfrac{11}{10}+\dfrac{9}{10}=\dfrac{20}{10}=2\)

\(\dfrac{11}{12}+\dfrac{5}{12}=\dfrac{16}{12}=\dfrac{4}{3}\)

\(\dfrac{7}{9}+\dfrac{5}{9}=\dfrac{12}{9}=\dfrac{4}{3}\)

17/18

13/6

30/28

16/5

5/9 + 7/18 = 10/18 + 7/18 = 17/18
3 - 5/6= 18/6 - 5/6 = 13/6
5/4 x 6/7 = 15/14
2 : 5/8 = 2/1 x 8/5 = 16/5

c: Đặt \(A=\frac{1}{51}+\frac{1}{52}+\cdots+\frac{1}{100}\)

Ta có: \(\frac{1}{51}>\frac{1}{75};\frac{1}{52}>\frac{1}{75};..;\frac{1}{75}=\frac{1}{75}\)

Do đó: \(\frac{1}{51}+\frac{1}{52}+\cdots+\frac{1}{75}>\frac{1}{75}+\frac{1}{75}+\cdots+\frac{1}{75}=\frac{25}{75}=\frac13\) (2)

Ta có: \(\frac{1}{76}>\frac{1}{100};\frac{1}{77}>\frac{1}{100};\ldots;\frac{1}{100}=\frac{1}{100}\)

Do đó: \(\frac{1}{76}+\frac{1}{77}+\cdots+\frac{1}{100}>\frac{1}{100}+\frac{1}{100}+\cdots+\frac{1}{100}=\frac{25}{100}=\frac14\) (1)

Từ (1),(2) suy ra \(\frac{1}{51}+\frac{1}{52}+\cdots+\frac{1}{75}+\frac{1}{76}+\frac{1}{77}+\cdots+\frac{1}{100}>\frac13+\frac14\)

=>\(A>\frac{7}{12}\)

=>\(\frac{1}{51}+\frac{1}{52}+\cdots+\frac{1}{100}>\frac{7}{12}\)

=>\(\frac{1}{50}+\frac{1}{51}+\frac{1}{52}+\cdots+\frac{1}{100}>\frac{7}{12}+\frac{1}{50}>\frac{7}{12}\) (ĐPCM)

a: \(\frac{1}{26}>\frac{1}{50};\frac{1}{27}>\frac{1}{50};\ldots;\frac{1}{50}=\frac{1}{50}\)

Do đó: \(\frac{1}{26}+\frac{1}{27}+\cdots+\frac{1}{50}>\frac{1}{50}+\frac{1}{50}+\cdots+\frac{1}{50}=\frac{25}{50}=\frac12\) (1)

Ta có: \(\frac{1}{26}<\frac{1}{25};\frac{1}{27}<\frac{1}{25};\ldots;\frac{1}{50}<\frac{1}{25}\)

Do đó: \(\frac{1}{26}+\frac{1}{27}+\cdots+\frac{1}{50}<\frac{1}{25}+\frac{1}{25}+\cdots+\frac{1}{25}=\frac{25}{25}=1\) (2)

Từ (1),(2) suy ra \(\frac12<\frac{1}{26}+\frac{1}{27}+\cdots+\frac{1}{50}<1\) (ĐPCM)

\(5\dfrac{5}{11}-\left(2\dfrac{3}{5}\cdot4\dfrac{6}{11}\right)=\dfrac{60}{11}-\left(\dfrac{13}{5}\cdot\dfrac{50}{11}\right)=\dfrac{60}{11}-\dfrac{130}{11}=-\dfrac{70}{11}\)

= 70/11

2/5*7+2/7*9+2/9*11+...+2/99*101

= 2*(1/5*7 + 1/7*9 + 1/9*11 + ... + 1/99*101)

= 2*( 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/99 - 1/101)

= 2*(1/5 - 1/101)

= 2*96/505

=192/505

tick mình nha

HT

3/9