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a)=1/2 . 8/15 - 3/4.47/9
=4/15 - 47/12
=-73/20
b)=2-1/3 . -21/20
=2+7/20
=47/20
![](https://rs.olm.vn/images/avt/0.png?1311)
17 + 15 : [(15 - x.3) -4] -11 = 9
17 + 15 : [(15 - x.3) -4] = 9 + 11
17 + 15 : [(15 - x.3) -4] = 20
15 : [(15 - x.3) -4] = 20 - 17
15 : [(15 - x.3) -4] = 3
(15 - x.3) -4 = 15 : 3
(15 - x.3) -4 = 5
15 - x.3 = 5 + 4
15 - x.3 = 9
x.3 = 15 - 9
x.3 = 6
x = 6 : 3
x = 2.
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Ta có: \(\frac{-3}{4}=\frac{-6}{x}\Rightarrow x=\frac{4.\left(-6\right)}{-3}=8\)
Thế x = 8 \(\Rightarrow\frac{-6}{8}=\frac{y}{20}\Rightarrow y=\frac{\left(-6\right).20}{8}=-15\)
Thế y = -15 \(\Rightarrow\frac{-15}{20}=\frac{-15}{z}\Rightarrow z=\frac{20.\left(-15\right)}{-15}=20\)
Vậy x = 8 ; y = -15 ; z = 20
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\(A=7+9+11+...+99\)(có (99-7):2+1=47 số hạng)
\(A=\frac{47\left(99+7\right)}{2}=\frac{47\cdot106}{2}=47\cdot53=2491\)
\(B=10+15+20+...+200\)(có (200-10):5+1=39 số hạng)
\(B=\frac{39\left(200+10\right)}{2}=\frac{39\cdot210}{2}=39\cdot105=18585\)
20-(15+20)=3-(x-4)
20-35=3-(x-4)
-15=3-(x-4)
3-(-15)=x-4
18=x-4
x=18+4
x=22
k chi minh nhe
20 - ( 15 + 20 ) = 3 - ( x - 4 )
20 - 35 = 3 - ( x - 4 )
3 - ( x - 4 ) = -15
x - 4 = 3 - ( -15 )
x - 4 = 18
x = 18 + 4
x = 22.