\(C\text{ó}:2+4+6+8+...+2x=210\)
\(\Leftrightarrow2.1+2.2+2.3+2.4+...+2.x=210\)
\(\Leftrightarrow2\left(1+2+3+4+...+x\right)=210\)
\(\Leftrightarrow1+2+3+4+...+x=105\)
\(\Rightarrow\frac{x\left(x+1\right)}{2}=105\)
\(\Rightarrow x\left(x+1\right)=210\)
\(14.15=210\)
\(\Rightarrow x=14\)
