tính A=47.28/3.215.162_5.22."210"2
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Dễ
a+20=120
a = 120-20
a =100
210+a=300
a =300-210
a =90
a+210=420
a =420-210
a =210
Gửi bạn nhé, T I C K cho mình nhá chắc chắn đúng 100% luôn.
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A = \(\dfrac{1}{2}\) + \(\dfrac{1}{6}\) + \(\dfrac{1}{12}\) + \(\dfrac{1}{20}\) + \(\dfrac{1}{30}\)+...+\(\dfrac{1}{182}\)+ 210
A = \(\dfrac{1}{1\times2}\) + \(\dfrac{1}{2\times3}\)+\(\dfrac{1}{3\times4}\)+ \(\dfrac{1}{4\times5}\)+...+ \(\dfrac{1}{13\times14}\)+ 210
A = \(\dfrac{1}{1}\) - \(\dfrac{1}{2}\)+ \(\dfrac{1}{2}\) - \(\dfrac{1}{3}\)+ \(\dfrac{1}{3}\)- \(\dfrac{1}{4}\)+ \(\dfrac{1}{4}\) - \(\dfrac{1}{5}\)+...+\(\dfrac{1}{13}\) - \(\dfrac{1}{14}\) + 210
A = 1 - \(\dfrac{1}{14}\) + 210
A = 211 - \(\dfrac{1}{14}\)
A = \(\dfrac{2953}{14}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(A=2+2^2+2^3+...+2^{10}\)
\(\Leftrightarrow A=\left(2+2^2\right)+\left(2^3+2^4\right)+..+\left(2^9+2^{10}\right)\)
\(\Leftrightarrow A=6+2^2\left(2+2^2\right)+..+2^8\left(2+2^2\right)\)
\(\Leftrightarrow A=6+2^2.6+...+2^8.6\)
\(\Leftrightarrow A=6\left(1+2^2+...+2^8\right)\)
Vì \(6⋮3\)
\(\Rightarrow A=6\left(1+2^2+..+2^8\right)⋮3\)
Vậy \(A⋮3\)
hok tốt !!!
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)