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Ta có: \(x+2x+3x+-+20x=210\)
\(\Leftrightarrow x\left(1+2+3+...+20\right)=210\)
\(\Leftrightarrow\dfrac{20\cdot\left(20+1\right)}{2}x=210\)
\(\Rightarrow x=\dfrac{210\cdot2}{20\cdot\left(20+1\right)}=1\)
Vậy \(x=1\)
