+26388 determine the sum of all the multiples of 4 between 1 and 999
arithmetical sequence: \(sum = 4 + 8 + 12 + 16 + 20 + 24 + \dots+996\)
\(n = \frac{996}{4} = 249\)
\(\begin{array}{|rcll|} \hline sum &=& \frac{(a_1+a_n)\cdot n}{2} \\ &=& \frac{(4+996)\cdot 249}{2} \\ &=& \frac{1000\cdot 249}{2} \\ &=& 500 \cdot 249 \\ &=& 124500 \\ \hline \end{array}\)
