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a) \(\dfrac{-15}{4}:\dfrac{21}{-10}=\dfrac{-15}{4}.\dfrac{-10}{21}=\dfrac{25}{14}\)
b) \(\dfrac{-7}{14}:\left(-0,14\right)=\dfrac{-7}{14}.\dfrac{-50}{7}=\dfrac{25}{7}\)
c) \(\left(\dfrac{-11}{15}\right):1\dfrac{1}{10}=\dfrac{-11}{15}.\dfrac{10}{11}=\dfrac{-2}{3}\)
d) \(2\dfrac{1}{7}:1\dfrac{1}{14}=\dfrac{15}{7}.\dfrac{14}{15}=2\)
\(a.-\dfrac{15}{4}:\left(\dfrac{21}{-10}\right)\)
\(=-\dfrac{15}{4}\cdot\left(-\dfrac{10}{21}\right)\)
\(=\dfrac{25}{14}\)
\(b.-\dfrac{7}{14}:\left(-0,14\right)\)
\(=-\dfrac{1}{2}:\left(-\dfrac{7}{50}\right)\)
\(=\dfrac{25}{7}\)
\(c.\left(-\dfrac{11}{15}\right):\left(1\dfrac{1}{10}\right)\)
\(=\left(-\dfrac{11}{15}\right):\dfrac{11}{10}\)
\(=-\dfrac{2}{3}\)
\(d.\left(2\dfrac{1}{7}\right):\left(1\dfrac{1}{14}\right)\)
\(=\dfrac{15}{7}:\dfrac{15}{14}\)
\(=2\)
![](https://rs.olm.vn/images/avt/0.png?1311)
c)đề sai ở phân số cuối
D ) 1/11×16 +1/16×21+...+1/61×66
Đặt Thắng=1/11×16 +1/16×21+...+1/61×66
5Thắng=5(1/11×16 +1/16×21+...+1/61×66)
5Thắng=1/11-1/16+1/16-1/21+...+1/61-1/66
5Thắng=1/11-1/66
Thắng=5/176:5=1/176
D) Đặt D=1/11×16 +1/16×21+...+1/61×66
=>5D=5.(1/11x16+1/16x21+...+1/61x66)
=>5D=5/11x16+5/16x21+...+5/61x66
=>5D=1/11-1/16+1/16-1/21+...+1/61-1/66
=>5D=1/11-1/66
=>5D=5/66
=>D=1/66
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\(A=\dfrac{1}{5}\left(\dfrac{1}{6}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}+...+\dfrac{1}{496}-\dfrac{1}{501}\right)\)
\(=\dfrac{1}{5}\cdot\dfrac{55}{334}=\dfrac{11}{334}\)
\(B=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+...+\dfrac{1}{19}-\dfrac{1}{21}=\dfrac{20}{21}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(-\dfrac{3}{17}+\left(\dfrac{2}{3}+\dfrac{3}{17}\right)=\dfrac{-3}{17}+\dfrac{3}{17}+\dfrac{2}{3}=\dfrac{2}{3}\)
\(\dfrac{-5}{21}+\dfrac{-16}{21}+1=-1+1=0\)
\(\dfrac{-4}{11}\cdot\dfrac{5}{15}\cdot\dfrac{11}{-4}=\dfrac{5}{15}=\dfrac{1}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
=16/11- (1/3+1/15+...+1/99)
=16/11- (1/1.3+1/3.5+...+1/9.11)
=16/11- 1/2(1/1-1/3+1/3-1/5+...+1/9-1/11)
=16/11 - 1/2(1-1/11)
=16/11 - 1/2. 10/11
=16/11-5/11 = 11/11 =1