20 | = | 12 |
41 - X | 21 |
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a 25 phần 41
b 9
c 1 phần 12
d 12
e 14 phần 15
f 24 phần 7
![](https://rs.olm.vn/images/avt/0.png?1311)
5.\(-\dfrac{3}{7}+\dfrac{5}{13}+\dfrac{-4}{12}=-\dfrac{103}{273}\)
b.\(-\dfrac{5}{21}+\dfrac{-2}{21}+\dfrac{8}{24}=\dfrac{-5-2}{21}+\dfrac{8}{24}=-\dfrac{7}{21}+\dfrac{8}{24}=-\dfrac{1}{3}+\dfrac{8}{24}=0\)
c.\(\dfrac{5}{13}+\dfrac{-5}{7}+\dfrac{-20}{41}+\dfrac{8}{13}+\dfrac{-21}{41}=\left(\dfrac{5}{13}+\dfrac{8}{13}\right)+\left(\dfrac{-20}{41}+\dfrac{-21}{41}\right)+-\dfrac{5}{7}=1-1-\dfrac{5}{7}=-\dfrac{5}{7}\)
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2-4+6-8+10-12+14-16+18-20+22
=(2+18)-(4+16)+(6+14)-(8+12)-(10+20)+22
=20-20+20-20-20+22
=22-20
=2
![](https://rs.olm.vn/images/avt/0.png?1311)
a,= 32.(11+67) +78.68
32.78+78.68
78.(32+68)
78.100
= 7800
b,76.(41-20)+21.24
=76.21+21.24
=21.(76+24)
=21.100
=2100
![](https://rs.olm.vn/images/avt/0.png?1311)
Khó quá thì nên hoc24 .vn
Nếu đúng thì ấnĐúng 2 không những thees sau khi ấn sẽ may mắn cả năm
![](https://rs.olm.vn/images/avt/0.png?1311)
A=1+1/2+1+1/6+1+1/12+...+1+1/90=
=9+1/2+1/6+1/12+...+1/90
1/2+1/6+1/12+...+1/90=
1/1x2+1/2x3+2/3x4+...+1/9x10=
\(=\dfrac{2-1}{1x2}+\dfrac{3-2}{2x3}+\dfrac{4-3}{3x4}+...+\dfrac{10-9}{9x10}=\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}=\)
\(=1-\dfrac{1}{10}=\dfrac{9}{10}\)
\(\Rightarrow A=9+\dfrac{9}{10}=9\dfrac{9}{10}\)
suy ra: 20 x 21 = 12 x ( 41- x)
420 = 492 -12x
12x= 492-420
12x= 72
x= 72 ;12
x = 6
vậy x=6
\(\dfrac{20}{41-x}=\dfrac{12}{21}\)
\(41-x=20:\dfrac{12}{21}\)
\(41-x=20\times\dfrac{21}{12}\)
\(41-x=\dfrac{420}{12}\)
\(41-x=35\)
\(x=41-35\)
\(x=6\)
Vậy \(=6\)