MATH · CALENDAR
Pnk=n!/(n−k)!, Cnk=n!/(k!(n−k)!)P_n^k=n!/(n-k)!,\; C_n^k=n!/(k!(n-k)!)Pnk=n!/(n−k)!,Cnk=n!/(k!(n−k)!)
从n中取k的排列和组合.
