1 + 5 + 9 + ........... + 81 = ?

প্রশ্ন: 1 + 5 + 9 + ........... + 81 = ?

সমাধান:
সমষ্টি = গড় × পদ সংখ্যা
= {(81 + 1)/2} × [{(81 - 1)/4} + 1]
= (82/2) × {(80/4) + 1}
= (82/2) × (20 + 1)
= 41 × 21
= 861