13/19.1/12+13/19.21/12-13/19.2/3
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9/13 x 7/12 + 9/13 x 5/12 - 9/13
= 9/13 x (7/12 + 5/12 - 1)
= 9/13 x 0
= 0
4/13 x 5/12 + 4/13 x 7/12 - 4/3
= 4/13 x (5/12 + 7/12) - 4/3
= 4/13 x 1 - 4/3
= 4/13 - 4/3
= -40/39
![](https://rs.olm.vn/images/avt/0.png?1311)
1) = \(\frac{3}{5}\)
2) =\(\frac{6}{7}\)
3)\(\frac{9}{13}\)
4)\(\frac{4}{13}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1/13+2/13+....+12/13
=1+2+3+..+12/13
=(12+1)x12:2/13
=13x12:2/13
=156:2/13
=78/13=6
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Câu a đề sai rồi bạn
b: \(3+3^2+3^3+...+3^{16}\)
\(=3\left(1+3\right)+3^3\left(1+3\right)+...+3^{15}\left(1+3\right)\)
\(=4\left(3+3^3+...+3^{15}\right)\)
\(=12\left(1+3^2+...+3^{14}\right)⋮12\)
![](https://rs.olm.vn/images/avt/0.png?1311)
=(1/13+12/13)+(2/13+11/13)+(3/13+10/13)+......(6/13+7/13)
=1+1+1+1+1+1
=6
\(=\frac{1+2+3+...+12}{13}\)
\(=\frac{\left(12+1\right)+\left(11+2\right)+..+\left(5+7\right)+6}{13}\)
\(=\frac{12+12+12+12+12+6}{13}\)
\(=\frac{12.5+6}{13}\)
\(=\frac{66}{13}\)
\(\frac{13}{19}\cdot\frac{1}{12}+\frac{13}{19}\cdot\frac{21}{12}-\frac{13}{19}\cdot\frac{2}{3}\)
\(=\frac{13}{19}\cdot\left(\frac{1}{12}+\frac{21}{12}-\frac{2}{3}\right)\)
\(=\frac{13}{19}\cdot\frac{7}{6}\)
\(=\frac{91}{114}\)