(2n)A4＝120×nC2(2n)!/(2n-4)!=120·n!/(2!·(n-2)!)(2n-3)(2n-2)(2n-1)·2n=60(n-1)n(2n-3)(2n-1)=154n^2-8n+3=15n^2-2n-3=0n=3
