72:4=?
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a) `1/2 xx 5/4 + 1/2 xx 3/4 + 1/2`
`=1/2 xx (5/4 + 3/4 + 1)`
`=1/2 xx 3`
`=3/2`
b) `72/95 xx 64/72 xx 95/72 xx 14/86 xx 72/64 xx 86/14`
`=(72xx64xx95xx14xx72xx86)/(95xx72xx72xx86xx64xx14)`
`= 1`.
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a,C1:\left(524-168\right):4=356:4=89\\ C2:\left(524-168\right):4=524:4-168:4=131-42=89\)
\(b,C1:72:\left(9\times4\right)=72:36=2\\C2:72:\left(9\times4\right)=\left(72:9\right):4 =8:4=2\)
\(c,C1:\left(25\times72\right):9=1800:9=200\\ C2:\left(25\times72\right):9=25\times\left(72:9\right)=25\times8=200\)
a,C1:(524−168):4=356:4=89C2:(524−168):4=524:4−168:4=131−42=89a,C1:(524−168):4=356:4=89C2:(524−168):4=524:4−168:4=131−42=89
b,C1:72:(9×4)=72:36=2C2:72:(9×4)=(72:9):4=8:4=2b,C1:72:(9×4)=72:36=2C2:72:(9×4)=(72:9):4=8:4=2
c,C1:(25×72):9=1800:9=200C2:(25×72):9=25×(72:9)=25×8=200
![](https://rs.olm.vn/images/avt/0.png?1311)
a. \(\frac{4}{25}\times72+\frac{4}{25}\times28=\frac{4}{25}\times\left(72+28\right)=\frac{4}{25}\times100=4\times4=16\)
b.\(0.5\times72+\frac{3}{6}\times28=0.5\times72+0.5\times28=0.5\times\left(72+28\right)=0.5\times100=50\)
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1) Ta có : 7245 - 7243 = 7243.(722 - 1)
7244 - 742 = 742.(722 - 1)
Vì 7243 > 7242
=> 7243.(722 - 1) > 742.(722 - 1)
=> 7245 - 7243 > 7244 - 742
2) Giải
\(M=\frac{1}{4}+\frac{1}{4^2}+\frac{1}{4^3}+....+\frac{1}{4^{50}}\)
\(4M=1+\frac{1}{4}+\frac{1}{4^2}+...+\frac{1}{4^{49}}\)
Lấy 4M trừ M theo vế ta có :
\(4M-M=\left(1+\frac{1}{4}+\frac{1}{4^2}+...+\frac{1}{4^{49}}\right)-\left(\frac{1}{4}+\frac{1}{4^2}+\frac{1}{4^3}+...+\frac{1}{4^{50}}\right)\)
\(3M=1-\frac{1}{49}\)
\(M=\left(1-\frac{1}{49}\right):3\)
\(=\frac{1}{3}-\frac{1}{147}< \frac{1}{3}\)
Vậy \(M< \frac{1}{3}\left(\text{đpcm}\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Đặt : A = 1 + 22 + 23 + 24 + .... + 29
=> 2A = 22 + 23 + 24 + .... + 210
=> 2A - A = 210 - 1
=> A = 210 - 1
Đặt B = 5 x 28 = (22 + 1) x 28 = 210 + 28 > 210 - 1
Vậy A < B .
18
72 : 4 = 18