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Đặt A=\(\dfrac{4^2}{20.24}+\dfrac{4^2}{24.28}+...+\dfrac{4^2}{76.80}\)
A=\(\dfrac{16}{20.24}+\dfrac{16}{24.28}+...+\dfrac{16}{76.80}\)
A=4.[\(\dfrac{4}{20.24}+\dfrac{4}{24.28}+...+\dfrac{4}{76.80}\)]
A=4.\(\left[\dfrac{1}{20}-\dfrac{1}{24}+\dfrac{1}{24}-\dfrac{1}{28}+...+\dfrac{1}{76}-\dfrac{1}{80}\right]\)
A=4.\(\left[\dfrac{1}{20}+\dfrac{1}{24}-\dfrac{1}{24}+\dfrac{1}{28}-\dfrac{1}{28}+...+\dfrac{1}{78}-\dfrac{1}{78}-\dfrac{1}{80}\right]\)
A=4.\(\left[\dfrac{1}{20}-\dfrac{1}{80}\right]\)
A=4.\(\dfrac{3}{80}\)
A=\(\dfrac{3}{20}\)<1
=>A<1
Tick mink nha
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(\frac{4^2}{20.24}+\frac{4^2}{24.28}+...+\frac{4^2}{76.80}\)
\(=4.\left(\frac{4}{20.24}+\frac{4}{24.28}+...+\frac{4}{76.80}\right)\)
\(=4.\left(\frac{1}{20}-\frac{1}{24}+\frac{1}{24}-\frac{1}{28}+...+\frac{1}{76}-\frac{1}{80}\right)\)
\(=4.\left(\frac{1}{20}-\frac{1}{80}\right)=4.\frac{3}{80}=\frac{3}{20}< 1\)
Vậy \(\frac{4^2}{20.24}+\frac{4^2}{24.28}+...+\frac{4^2}{76.80}< 1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
28 x 76 + 24 x 28 - 28 x 20
= 28 x (76 + 24 - 20)
= 28 x 80
= 2240
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có : 28 . 76 + 24 . 28 - 28 . 20 = 28 . ( 76 + 24 - 20 )
= 28 . 80
= 2240
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Giải cho bạn 2 bài tìm x nhé! Bài kia dễ,tự giải
a) \(3^{x+2}-3^x=8.243\Leftrightarrow3^x.3^2-3^x=1944\)
\(\Leftrightarrow3^x\left(3^2-1\right)=1944\Leftrightarrow3^x.8=1944\)
\(\Leftrightarrow3^x=243=3^5\Leftrightarrow x=5\)
b) \(1+2+3+...+x=210\)
\(\Leftrightarrow\frac{x\left(x+1\right)}{2}=210\Leftrightarrow x\left(x+1\right)=420\)
\(\Leftrightarrow20\left(20+1\right)=420\Rightarrow x=20\)
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a, 160-(8.25-6.25)
= 160-[25.(8-6)]
=160-(25.2)
=160-50
=110
b, 28.(76+24)
=28.100
=2800
a. 160 - ( 23 . 52 - 6 .25 )
= 160 - ( 8 . 25 - 6 .25 )
= 160 - [ 25 . ( 8 - 6 ) ]
= 160 - [ 25 . 2 ]
= 160 - 50 = 110
b. 28 . 76 + 24 . 28
= 28 . ( 76 + 24 )
= 28 . 100 = 2800
c. 4 . 52 - 32 . 24
= 4 . 25 - 32 . 16
= 4 . 25 - 32 . 4 . 4
= 4 . ( 25 - 32 . 4 )
= 4 . ( 25 - 128 )
= 4 . ( - 103 ) = - 412
d. 587 : [ 928 - ( 247 - 82 ) . 5 ]
= 587 : [ 928 - 165 . 5 ]
= 587 : [ 928 - 825 ]
= 587 : 103 = 5.69902913
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a) 7 . 52 - 6 . 42
= 7 . 25 - 6 . 16
= 175 - 96
= 79
b) 16 . 24 + 76 . 16 + (-1600)
= 16 . (24+76) + (-1600)
= 16 . 100 + (-1600)
= 1600 + (-1600)
= 0
#Hok_tốt
\(a,7\times5^2-6\times4^2=\)
\(=7\times25-6\times16\)
\(=175-96\)
\(=79\)
\(b,16\times24+76\times16+\left(-1600\right)=\)
\(=16\times\left(24+76\right)+\left(-1600\right)\)
\(=16\times100+\left(-1600\right)\)
\(=1600+\left(-1600\right)\)
\(=0\)
4 x 7 x 76 + 28 x 24
= 28 x 76 + 28 x 24
= 28 x (76 + 24)
= 28 x 100
= 2800
\(4.7.76+28.24\)
\(=28.76+28.24\)
\(=28.\left(76+24\right)\)
\(=28.100\)
\(=2800\)