\(2+4+6+8+...+2x=210\)
\(\Rightarrow1+2+3+4+...+x=105\)
\(\Rightarrow\dfrac{x.\left(1+x\right)}{2}=105\)
\(\Rightarrow x.\left(1+x\right)=210\)
\(\Rightarrow x=14\)
Vậy x = 14.
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