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231 - ( x - 6 ) = 1339 : 13
231 - ( x - 6 ) = 103
x = 231 - 103 + 6
x = 128 + 6
x = 134
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1/2 + 1/6+1/12 + 1/20 +....+ 1/x(x+1) = 2021/2022
1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 +...+ 1/x. (x+1) = 2021/2020
1 - 1/2 + 1/2 - 1/3 + 1/3- 1/4 + 1/4 - 1/5 +...+ 1/x - 1/(x+1) = 2021/2020
1 - 1/(x+1) = 2021/2020
1/(x+1) = 1 - 2021/2020
1/(x+1) = -1/2020
1/(x+1) = 1/-2020
x + 1 = - 2020
x = -2020 - 1
x = -2021
Giải:
1/2+1/6+1/12+1/20+...+1/x.(x+1)=2021/2022
1/1.2+1/2.3+1/3.4+1/4.5+...+1/x.(x+1)=2021/2022
1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/x-1/x+1=2021/2022
1/1-1/x+1 =2021/2022
1/x+1 =1/1-2021/2022
1/x+1 =1/2022
⇒x+1=2022
x=2022-1
x=2021
Chúc bạn học tốt!
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Bài 1 :
\(=\dfrac{2}{11}+\dfrac{4}{11}-\dfrac{6}{11}-\dfrac{3}{8}-\dfrac{5}{8}=0-1=-1\)
Bài 2 :
\(\Rightarrow3+x=8\Leftrightarrow x=5\)
Bài 3 :
\(\Leftrightarrow x-\dfrac{5}{11}=\dfrac{5}{4}\Leftrightarrow x=\dfrac{35}{44}\)
Bài 4 :
Trong 2 ngày An đọc được số quyên phần quyên sách
\(\dfrac{1}{11}+\dfrac{8}{11}=\dfrac{9}{11}\)( quyển sách )
đs : 9/11 quyển sách
![](https://rs.olm.vn/images/avt/0.png?1311)
( 2 x y + 2/15 ) x 3 = 4/5
( 2 x y + 2/15 ) = 4/5 : 3
( 2 x y + 2/15 ) = 4/15
2 x y = 4/15 - 2/15
2 x y = 2/15
y = 2/15 :2
y = 1/15
(2 x y + 2/15) x 3 = 4/5
2 x y + 2/15) = 4/5 : 3
2 x y + 2/15 = 4/15
2 x y = 4/15 - 2/15
2 x y = 2/15
y = 2/15 : 2
y = 1/15
7/9 x (2 - 1/3 x y) = 14/15
(2 - 1/3 x y) = 14/15 : 7/9
(2 - 1/3 x y) = 6/5
2 - y = 6/5 x 1/3
2 - y = 2/5
y = 2/5 + 2
y = 12/5
4/21 + 5 x y - 8/7 = 1/3
4/21 + 5 x y = 1/3 + 8/7
4/21 + 5 x y = 31/21
5 x y = 31/21 - 4/21
5 x y = 9/7
y = 9/7 : 5
y = 9/35
7/12 x y - 3/12 x y = 5
y x (7/12 - 3/12) = 5
y x 1/3 = 5
y = 5 : 1/3
y = 15
-94 x 12 + (-2) x 6 x (-5) + 12 x (-1)
= -94 x 12 + 12 x 5 + 12 x (-1)
= 12 x (-94 + 5 - 1)
= 12 x (-90)
= 1080.