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Ta thấy:
\(\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+...+\frac{1}{17\cdot19}\)
\(=\frac{1}{2}\cdot\frac{2}{3\cdot5}+\frac{1}{2}\cdot\frac{2}{5\cdot7}+...+\frac{1}{2}\cdot\frac{2}{17\cdot19}\)
\(=\frac{1}{2}\cdot\left(\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+...+\frac{2}{17\cdot19}\right)\)
\(=\frac{1}{2}\cdot\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{17}-\frac{1}{19}\right)\)
\(=\frac{1}{2}\cdot\left[\frac{1}{3}-\left(\frac{1}{5}-\frac{1}{5}\right)-\left(\frac{1}{7}-\frac{1}{7}\right)-...-\left(\frac{1}{17}-\frac{1}{17}\right)-\frac{1}{19}\right]\)
\(=\frac{1}{2}\cdot\left(\frac{1}{3}-0-0-...-0-\frac{1}{19}\right)\)
\(=\frac{1}{2}\cdot\left(\frac{1}{3}-\frac{1}{19}\right)\)
\(=\frac{1}{2}\cdot\left(\frac{19}{57}-\frac{3}{57}\right)\)
\(=\frac{1}{2}\cdot\frac{16}{57}\)
\(=\frac{1\cdot16}{2\cdot57}\)
\(=\frac{8}{57}\)
Khi đó:
\(\left(\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+...+\frac{1}{17\cdot19}\right)\cdot114-0,2\cdot\left(x-1\right)=10\)
\(\Rightarrow\frac{8}{57}\cdot114-\frac{1}{5}\cdot\left(x-1\right)=10\)
\(\Rightarrow8\cdot2-\left(x-1\right)\cdot\frac{1}{5}=10\)
\(\Rightarrow16-\frac{x-1}{5}=10\)
\(\Rightarrow\frac{x-1}{5}=16-10\)
\(\Rightarrow\frac{x-1}{5}=6\)
\(\Rightarrow x-1=6\cdot5\)
\(\Rightarrow x-1=30\)
\(\Rightarrow x=30+1\)
\(\Rightarrow x=31\)
Vậy x = 31
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a, 3 1\3 : 2 1\2 - 1 < x < 7 2\3 . 3\7 + 5\2
10\3 : 5\2 - 1 < x < 23\3 . 3\7 + 5\2
4\3 - 1 < x < 23\7 + 5\2
1\3 < x < 81\14
Ta thấy 1\3 < 1 và 81\14 > 5 suy ra :
Tập hợp X = {1;2;3;4;5}
b,1/2 - (1/3 + 1/4) < x < 1\48 - ( 1\16 - 1\16)
1/2 - 7/12 < x < 1\48 - 0
-1/12 < x < 1/48
Vì -1/12 < 0 và 1/48 > 0 suy ra :
Tập hợp X = {0}
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a)=1/2*2/3......*19/20
=1/20
b)=3/2*4/3......*2008/2007
=3/2007
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b) ( x + 1 ) + ( x +2 ) + ( x + 3 ) + ..... + ( x + 99 ) =6130
( x + x + ... + x ) + ( 1 + 2 + 3 +... + 99 ) =6130
99 × x + 4950 =6130
99 × x = 6130 - 4950
99 × x = 1175
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Câu 1:
0,9 x 218 x 2 + 0,18 x 4290 + 0,6 x 353 x 3
= 9/10 x 436 + 9/50 x 4290 + 6/10 x 1059
= 9 x 43,6 + 9 x 85,8 + 6 x 105,9
= 3 x 130,8 + 3 x 257,4 + 3 x 211,8
= 3 x ( 130,8 + 257,4 + 211,8 )
= 3 x 600
= 1800
Câu 2:
3/4 x X + 1/2 x X - 15 = 35
X x ( 3/4 + 1/2 ) - 15 = 35
X x ( 3/4 + 1/2 ) = 50
X x 5/4 = 50
X = 40
VẬy X = 40
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1 + 4 + 7 + 9 + 3 + 1 + 5 + 6 + 8 + 2 : 9
= 44 + \(\frac{2}{9}\)
= \(\frac{398}{9}\)
ủng hộ mk nha
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Nhân thêm 2
=>(13-11/11x13+...+21-19/19x21)-x+221/231=4/3
=>(13/11x13-11/11x13+...+21/19x21-19/19x21)-x+221/231=4/3
=>(1/11-1/13+...+1/18-1/21)-x+221/231=4/3
=>(1/11-1/21)-x+221/231=4/3
=>10/231-x+221/231=4/3
=>10/231+221/231-x=4/3
=>231/231-x=4/3
=>1-x=4/3
=>x=1-4/3=-1/3
Ta có : 1/2 + 1/3 < x < 3 - 1/3
=> 5/6 < x <8 /3
=> x = 1 ; 6/5 ; 7/5
giúp tớ nhé
tớ bị trừ tới 590 điểm
ủng hộ nhé
cảm ơn trước
tớ nhanh nhất
1/2+1/3<x<3-1/3
Giải hộ mk nhé
Ta có : 1/2 + 1/3 < x < 3 - 1/3
=> 5/6 < x <8 /3
=> x = 1 ; 6/5 ; 7/5