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a) |x| = 1212
=> x = 1212 hoặc -1212
T_T các câu kia tườn tự vậy thôi bạn, dài quá @_@
học tốt -_-"
a) \(\left|x\right|=1212\)
\(\Rightarrow\orbr{\begin{cases}x=1212\\x=-1212\end{cases}}\)
b) \(\left|2x+1212\right|=3434\)
\(\Rightarrow\orbr{\begin{cases}2x+1212=3434\\2x+1212=-3434\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}2x=2222\\2x=-4646\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}x=1111\\x=-2323\end{cases}}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1) -3/7 và 2/-5
Ta có -3/7= -15/35
2/-5= -14/35
mà -15< -14 nên -15/35< -14/35
Vậy -3/7< 2/-5
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\(B=1+\frac{1}{2}\left(1+2\right)+\frac{1}{3}\left(1+2+3\right)+...+\frac{1}{20}\left(1+2+3+...+20\right)\)
\(=1+\frac{1}{2}.\frac{2\left(2+1\right)}{2}+\frac{1}{3}.\frac{3\left(3+1\right)}{2}+...+\frac{1}{20}.\frac{20\left(20+1\right)}{2}\)
\(=\frac{2}{2}+\frac{2+1}{2}+\frac{3+1}{2}+...+\frac{20+1}{2}\)
\(=\frac{2}{2}+\frac{3}{2}+\frac{4}{2}+...+\frac{20}{2}\)
\(=\frac{2+3+4+...+20}{2}=\frac{\frac{20\left(20+1\right)}{2}-1}{2}=\frac{209}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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Ta có:
\(B=1+\dfrac{1}{2}\left(1+2\right)+\dfrac{1}{3}\left(1+2+3\right)+...+\dfrac{1}{20}\left(1+2+...+20\right)\)
\(=1+\dfrac{1}{2}.\dfrac{2\left(2+1\right)}{2}+\dfrac{1}{3}.\dfrac{3\left(3+1\right)}{2}+...+\dfrac{1}{20}.\dfrac{20\left(20+1\right)}{2}\)
\(=\dfrac{2}{2}+\dfrac{2+1}{2}+\dfrac{3+1}{2}+...+\dfrac{20+1}{2}\)
\(=\dfrac{2}{2}+\dfrac{3}{2}+\dfrac{4}{2}+...+\dfrac{20}{2}\)
\(=\dfrac{2+3+4+...+20}{2}=\dfrac{\dfrac{20\left(20+1\right)}{2}-1}{2}\)
\(=\dfrac{209}{2}\)
Vậy \(B=\dfrac{209}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(B=1+\frac{1}{2}\left(1+2\right)+\frac{1}{3}\left(1+2+3\right)+...+\frac{1}{20}\left(1+2+3+...+20\right)\)
\(=1+\frac{1}{2}.2.3:2+\frac{1}{3}.3.4:2+...+\frac{1}{20}.20.21:2\)
=\(\frac{2}{2}+\frac{3}{2}+\frac{4}{2}+...+\frac{21}{2}=\frac{2+3+4+...+21}{2}=\frac{230}{2}=115\)
=0 vì đó là các cặp số đối
chính xác 100%