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NN
2
6 tháng 9 2019
\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{49.50.51}\)
\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.5}+...+\frac{1}{49.50}-\frac{1}{50.51}\)
\(=\frac{1}{2}-\frac{1}{50.51}\)
\(=\frac{1}{2}-\frac{1}{2550}=\frac{637}{1275}\)
DT
6 tháng 9 2019
Gọi A là tổng dãy phân số trên
Ta có :
\(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{49.50.51}\)
\(2A=\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{49.50.51}\)
Ta thấy:
\(\frac{2}{1.2.3}=\frac{1}{1.2}-\frac{1}{2.3};\frac{2}{2.3.4}=\frac{1}{2.3}-\frac{1}{3.4};...;\frac{2}{49.50.51}=\frac{2}{49.50}-\frac{2}{50.51}\text{}\)
\(\Rightarrow2A=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{49.50}-\frac{1}{50.51}\)
\(\Rightarrow2A=\frac{1}{1.2}-\frac{1}{50.51}\)
\(\Rightarrow2A=\frac{1}{2}-\frac{1}{2550}\)
\(\Rightarrow2A=\frac{1275}{2550}-\frac{1}{2550}\)
\(\Rightarrow2A=\frac{637}{1275}\Rightarrow A=\frac{637}{1275}:2=\frac{637}{2550}\)
Vậy tổng dãy phân số trên là :\(\frac{637}{2550}\)
Chúc bạn học tốt !!! :D
S
15
12 tháng 6 2015
Coi \(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{98.99.100}\)
\(\Rightarrow2A=2x\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{98.99.100}\right)\)
\(=\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{98.99.100}\)
\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{98.99}-\frac{1}{99.100}\)
\(=\frac{1}{1.2}-\frac{1}{99.100}\)
\(=\frac{4950}{9900}-\frac{1}{9900}\)
\(=\frac{4949}{9900}\)
7 tháng 11 2016
a)
\(x.\frac{7}{9}=\frac{2}{3}+2\frac{1}{2}\)
\(x.\frac{7}{9}=\frac{19}{6}\)
\(x=\frac{19}{6}:\frac{7}{9}\)
\(x=\frac{57}{14}\)
b) \(\frac{5}{7}+x:\frac{9}{4}=\frac{4}{3}\)
\(x:\frac{9}{4}=\frac{4}{3}-\frac{5}{7}\)
\(x:\frac{9}{4}=\frac{13}{21}\)
\(x=\frac{13}{21}.\frac{9}{4}\)
\(x=\frac{39}{28}\)
BB
4
14 tháng 5 2017
\(\left(x-\frac{1}{2}\right).\frac{5}{2}+\frac{1}{2}=\frac{7}{4}\)
\(\left(x-\frac{1}{2}\right).\frac{5}{2}=\frac{7}{4}-\frac{1}{2}\)
\(\left(x-\frac{1}{2}\right).\frac{5}{2}=\frac{7}{4}-\frac{2}{4}\)
\(\left(x-\frac{1}{2}\right).\frac{5}{2}=\frac{5}{4}\)
\(\left(x-\frac{1}{2}\right)=\frac{5}{4}:\frac{5}{2}\)
\(x-\frac{1}{2}=\frac{1}{2}\)
\(x=\frac{1}{2}+\frac{1}{2}\)
\(x=1\)
26 tháng 11 2016
\(a,97\times327+327\times3\)
\(=327\times\left(97+3\right)\)
\(=327\times100\)
\(=32700\)
\(b,\frac{1}{7}\times\frac{4}{23}+\frac{1}{7}\times\frac{25}{23}+\frac{1}{7}\times\frac{17}{23}+5\times\frac{1}{7}\)
\(=\frac{1}{7}\times\left(\frac{4}{23}+\frac{25}{23}+\frac{17}{23}+5\right)\)
\(=\frac{1}{7}\times7\)
\(=1\)
S
26 tháng 11 2016
a) 97 x 327 + 327 x 3
= (97 + 3) x 327
= 100 x 327 = 32700
b) \(\frac{1}{7}\times\frac{4}{23}+\frac{1}{7}\times\frac{25}{23}+\frac{1}{7}\times\frac{17}{23}+5\times\frac{1}{7}\)
\(=\frac{1}{7}\times\left(\frac{4}{23}+\frac{25}{23}+\frac{17}{23}+5\right)\)
\(=\frac{1}{7}\times7=\frac{7}{7}=1\)
11 tháng 6 2017
\(\left[\left(35\frac{5}{7}+2\frac{3}{4}\right)-5\frac{5}{7}+\frac{1}{4}\right]:\left(11+x\right)=3\)
\(\left[\left(35\frac{5}{7}-5\frac{5}{7}\right)+2\frac{3}{4}+\frac{1}{4}\right]:\left(11+x\right)=3\)
\(\left[30+3\right]:\left(11+x\right)=3\)
\(33:\left(11+x\right)=3\)
\(11+x=11\)
\(x=0\)
DP
11 tháng 6 2017
\(\left[\left(35\frac{5}{7}+2\frac{3}{4}\right)-5\frac{5}{7}+\frac{1}{4}\right]\div\left(11+x\right)=3\)
\(\left[\left(\frac{35\times7+5}{7}+\frac{2\times4+3}{4}\right)-\frac{5\times7+5}{7}+\frac{1}{4}\right]\div\left(11+x\right)=3\)
\(\left[\left(\frac{250}{7}+\frac{11}{4}\right)-\frac{40}{7}+\frac{1}{4}\right]\div\left(11+x\right)=3\)
\(\left[\frac{1077}{28}-\frac{167}{28}\right]\div\left(11+x\right)=3\)
\(32,5\div\left(11+x\right)=3\)
\(11+x=32,5\div3\)
\(11+x=\frac{65}{6}\)
\(x=\frac{65}{6}-11=-\frac{1}{6}\)
Y
0
TV
18 tháng 9 2017
a,
x.2/7.3/4=5/21
x.3/14=5/21
x=5/21:3/14
x=10/9
b,
x.1/2=1/3
x=1/3:1/2
x=2/3
c,
x:4/5=25/8:5/4
x:4/5=5/2
x=5/2.4/5=2
KC
18 tháng 9 2017
A, x= 5/25 : 3/4 : 2/7 = 14/15
B, x=1/3 : 1/2 = 2/3
C, x=(25/8 : 5/4)x4/5 = 5/2 x 4/5 = 2
\(\frac{4}{7}x+\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{12.13.14}\right)=\frac{39}{40}\)
\(\Leftrightarrow\frac{4}{7}x+\left[\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{12.13}-\frac{1}{13.14}\right)\right]=\frac{39}{40}\)
\(\Leftrightarrow\frac{4}{7}x+\left[\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{13.14}\right)\right]=\frac{39}{40}\)
\(\Leftrightarrow\frac{4}{7}x+\left[\frac{1}{2}.\frac{45}{91}\right]=\frac{39}{40}\)
\(\Leftrightarrow\frac{4}{7}x+\frac{45}{182}=\frac{39}{40}\)
\(\Leftrightarrow\frac{4}{7}x=\frac{39}{40}-\frac{45}{182}\Leftrightarrow\frac{4}{7}x=\frac{2649}{3640}\)
\(\Rightarrow x=\frac{2649}{3640}\div\frac{4}{7}=\frac{2649}{2080}\)
Vậy x = \(\frac{2649}{2080}\)