【Question 21】  
Xiaoming adds even numbers $2, 4, 6, \cdots$ in sequence until a certain number, but due to missing one even number during the calculation, he gets an incorrect result of 2014. The even number that was missed is $\qquad$.
【Analysis and Solution】
Arithmetic sequence.
Let the last even number be $2 n$;
Then, if there is no calculation error, the total sum is $2+4+6+\cdots+2 n=(2+2 n) \times n \div 2=n \times(n+1)$;
$$
\begin{array}{l}
44 \times(44+1)=19802014 ;
\end{array}
$$

The missing even number is $2070-2014=56$.