[질문] 펙토리알(!)의합증명이요ㅠㅠ

nPn=n*(n-1)*(n-2)*.....*1=n!이고,nPr=n!/(n-r)!이므로,이식에r->n을대입하면,nPn=n!/(n-n)!=n!/0!이되고,n!/0!=n!에서,0!=1