2,-4,6,-8,10,-12,14,-16,18,-20.这组数列中,第999个是什么?

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2,-4,6,-8,10,-12,14,-16,18,-20.这组数列中,第999个是什么?2,-4,6,-8,10,-12,14,-16,18,-20.这组数列中,第999个是什么?2,-4,6,-

2,-4,6,-8,10,-12,14,-16,18,-20.这组数列中,第999个是什么?
2,-4,6,-8,10,-12,14,-16,18,-20.这组数列中,第999个是什么?

2,-4,6,-8,10,-12,14,-16,18,-20.这组数列中,第999个是什么?
2*999=1998

数列通项公式为 a(n) = 2n * (-1)^ (n+1)
故,第999项为 a(999) = 2 * 999 * (-1)^1000 = 2 * 999 = 1998