15×15=?25×25=?35×35=? 15×15=〔1×（1+1）〕×100+25=225 25X25=〔2X（2+1）〕X100+25=625 35X35=〔3X（3+1）〕X100+25=1225

The arithmetic sequence an = 5 + 10 × (n-1) = 10n-5 505 = 10n-5 yields n = 51 and = (a1 + A51) × 51 / 2 = 13005 (if you add 500, the answer is 13005-5 = 1300)

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