tính nhanh
2 x 4 x 5 x 20 x 25 x 5
1 + 2 + 3 + .....+ 9 + 10
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=\(\dfrac{2}{3}\times\left(\dfrac{4}{9}+\dfrac{5}{9}\right)=\dfrac{2}{3}\times\dfrac{9}{9}=\dfrac{2}{3}\)
\(=\dfrac{2}{3}\times\left(\dfrac{4}{9}+\dfrac{5}{9}\right)\\ =\dfrac{2}{3}\times1=\dfrac{2}{3}\)
A) 7/38 x 9/11 +7/38 x 4/11 -7/38 x 2/11
=7/38.(9/11+4/11-2/11)
=7/38
B) 5/31 x 21/25 + 5/31 x -7/10 - 5/31 x 9/20
=5/31.(21/25-7/10-9/20)
=5/31.(-31/100)
=-1/20
1: =-2/9(15/17+2/17)=-2/9
2: \(=\dfrac{-6}{3}+\dfrac{-21}{90}\)
=-2-7/30=-67/30
3: \(=\dfrac{3}{4}\cdot\dfrac{7}{5}+\dfrac{9}{7}\cdot\dfrac{3}{2}\)
=21/20+27/14=417/140
4: =-25/13(5/19+14/19)=-25/13
5: =-7/5-45/21=-7/5-15/7=-124/35
1: =-2/9(15/17+2/17)=-2/9
2: =−63+−2190=−63+−2190
=-2-7/30=-67/30
3: =34⋅75+97⋅32=34⋅75+97⋅32
=21/20+27/14=417/140
4: =-25/13(5/19+14/19)=-25/13
5: =-7/5-45/21=-7/5-15/7=-124/35
d) \(x-\left(-25+x\right)=13-x.\)
\(\Rightarrow x+25+x=13-x\)
\(\Rightarrow x+x+x=13-25\)
\(\Rightarrow3x=-12\)
\(\Rightarrow x=-4\)
e) \(15-\left(30+x\right)=x-\left(27-\text{| }-8\text{| }\right)\)
\(\Rightarrow15-30-x=x-19\)
\(\Rightarrow-15-x=x-19\)
\(\Rightarrow-15+19=x+x\)
\(\Rightarrow4=2x\)
\(x=2\)
f) \(\left(12x-4^3\right).8^3=4.8^4\)
\(\left(12x-2^6\right).2^9=2^2.2^{12}\)
\(12x-2^6=2^2.2^{12}\div2^9\)
\(12x-64=2^5=32\)
\(12x=96\)
\(x=8\)
g) \(\left[119-\left(3^3-10\right)\right].x=2448\)
\(\left[119-\left(27-10\right)\right].x=2448\)
\(\left[119-17\right].x=2448\)
\(102.x=2448\)
\(x=24\)
h)
[( 10 - x ) .2-51] : 3 - 2 = 3
[( 10 - x ) .2-51] : 3 = 3 + 2
[( 10 - x ) .2-51] : 3 = 5
( 10 - x ) .2 - 51 = 5 . 3
( 10 - x ) . 2 - 51 = 15
( 10 - x ) . 2 = 15 + 51
( 10 - x ) . 2 = 66
10 - x = 66 : 2
10 - x = 33
x = 10 - 33
x = -23
i)
(x-12)-15 = (20-7)-(18+x)
x-12-15 = 20-7-18-x
x-27 = -5-x
=> x-27+5+x = 0
2x-25 =0
2x = 0+25 = 25
k)
a) 2.4.5.20.25.5=(2.5).(4.25).(5.20)=10.100.100=100000
b) số số hạng (10-1):1+1=10
tổng là (10+1).10:2=55
2.4.5.20.25.5 = 100000
1+2+3+.....+9+10=55