（1+2+3）+（2+3+4）+（3+4+5）+……（498+499+500)
= 1 + 2 * 2 + 3 * 3 + 4 * 3 + 5 * 3 + ... + 498 * 3 + 499 * 2 + 500
= (1 + 2 + 3 + ... + 499) * 3 - 1 * 2 - 2 * 1 - 499 * 1 + 500
= (1 + 499) * 499 / 2 * 3 - 2 - 2 + 1
= 250 * 499 * 3 - 3
= 374250 - 3
= 374247