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![](https://rs.olm.vn/images/avt/0.png?1311)
3 x 4+3 x 7/6 x 5 +9
=3 x(4+7)/6 x5 +9
=3 x 11/2 x 3 +9
=11/11=1
6 x 9 - 2 x 17/63 x 3 - 119
=2 x 3 x 9-2 x 17/7 x 9 x 3-119
=2 -2 x 17/7-7 x17
=2-2/7-7=0
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:\(\frac{1}{3x4}+\frac{2}{4x6}+\frac{3}{6x9}+\frac{4}{9x13}+\frac{5}{13x18}\)
\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}\)
\(=\frac{1}{3}-\frac{1}{18}=\frac{6}{18}-\frac{1}{18}=\frac{5}{18}\)
đặt \(A=\frac{1}{3.4}+\frac{2}{4.6}+\frac{3}{6.9}+\frac{4}{9.13}+\frac{5}{13.18}\)=\(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}=\frac{1}{3}-\frac{1}{18}=\frac{5}{18}\)
=>A=5/18
vậy........
![](https://rs.olm.vn/images/avt/0.png?1311)
a) \(\frac{{21}}{{13}} = \frac{{21.2}}{{13.2}} = \frac{{42}}{{26}}\)
b) \(\frac{{12}}{{ - 25}} = \frac{{12.3}}{{ - 25.3}} = \frac{{36}}{{ - 75}}\)
c) \(\frac{{18}}{{ - 48}} = \frac{{18:6}}{{ - 48:6}} = \frac{3}{{ - 8}}\)
d) \(\frac{{ - 42}}{{ - 24}} = \frac{{ - 42:(-6)}}{{ - 24:( - 6)}} = \frac{7}{4}\).
a: \(\dfrac{21}{13}=\dfrac{21\cdot2}{13\cdot2}=\dfrac{42}{26}\)
b: \(\dfrac{12}{-25}=\dfrac{12\cdot\left(-1\right)}{\left(-25\right)\cdot\left(-1\right)}=\dfrac{-12}{25}\)
c: \(\dfrac{18}{-48}=\dfrac{-18}{48}=\dfrac{-18:6}{48:6}=\dfrac{-3}{8}\)
d: \(\dfrac{-42}{-24}=\dfrac{42}{24}=\dfrac{42:6}{24:6}=\dfrac{7}{4}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta rút gọn các phân số về dạng tối giản:
Do vậy ta có:
Phân số không bằng phân số nào trong các phân số còn lại.
Ta rút gọn các phân số về dạng tối giản:
Do vậy ta có:
Phân số không bằng phân số nào trong các phân số còn lại.
![](https://rs.olm.vn/images/avt/0.png?1311)
a) Ta có: \(\frac{16}{15}\cdot\frac{-5}{14}\cdot\frac{54}{24}\cdot\frac{56}{21}\)
\(=\frac{16}{15}\cdot\frac{-5}{14}\cdot\frac{9}{4}\cdot\frac{8}{3}\)
\(=4\cdot\frac{-1}{3}\cdot\frac{4}{7}\cdot3\)
\(=12\cdot\frac{-4}{21}=\frac{-48}{21}=\frac{-16}{7}\)
b) Ta có: \(5\cdot\frac{7}{5}=\frac{35}{5}=7\)
c) Ta có: \(\frac{1}{7}\cdot\frac{5}{9}+\frac{5}{9}\cdot\frac{1}{7}+\frac{5}{9}\cdot\frac{3}{7}\)
\(=\frac{5}{9}\left(\frac{1}{7}+\frac{1}{7}+\frac{3}{7}\right)\)
\(=\frac{5}{9}\cdot\frac{5}{7}=\frac{25}{63}\)
d) Ta có: \(4\cdot11\cdot\frac{3}{4}\cdot\frac{9}{121}\)
\(=\frac{4\cdot11\cdot3\cdot9}{4\cdot121}=\frac{27}{11}\)
e) Ta có: \(\frac{3}{4}\cdot\frac{16}{9}-\frac{7}{5}:\frac{-21}{20}\)
\(=\frac{4}{3}+\frac{4}{3}=\frac{8}{3}\)
g) Ta có: \(2\frac{1}{3}-\frac{1}{3}\cdot\left[\frac{-3}{2}+\left(\frac{2}{3}+0,4\cdot5\right)\right]\)
\(=\frac{7}{3}-\frac{1}{3}\cdot\left[\frac{-3}{2}+\frac{2}{3}+2\right]\)
\(=\frac{7}{3}-\frac{1}{3}\cdot\frac{7}{6}\)
\(=\frac{7}{3}-\frac{7}{18}=\frac{42}{18}-\frac{7}{18}=\frac{35}{18}\)
\(\frac{3\cdot4+3\cdot7}{6\cdot5+9}=\frac{3\left(4+7\right)}{2\cdot3\cdot5+3\cdot3}=\frac{3\cdot11}{3\left(2\cdot5+3\right)}=\frac{3\cdot11}{3\cdot13}=\frac{11}{13}\)
\(\frac{6\cdot9-2\cdot17}{63\cdot3-119}=\frac{2\cdot3\cdot9-2\cdot17}{7\cdot9\cdot3-7\cdot17}=\frac{2\left(27-17\right)}{7\left(27-17\right)}=\frac{2}{7}\)
\(\frac{21\cdot3-21}{1-24}=\frac{21\left(3-1\right)}{-23}=\frac{21\cdot2}{-23}=\frac{42}{-23}=-\frac{42}{23}\)
\(\frac{9\cdot3-54}{18}=\frac{9\cdot3-9\cdot6}{9\cdot2}=\frac{9\cdot\left(3-6\right)}{9\cdot2}=\frac{3-6}{2}=-\frac{3}{2}\)