Решите задачу:
N!=1*2*3*....*n (2k+1)!/(2k-1)!=1*2*3*.....*(2k-1)*2k(2k+1)/1*2*....*(2k-1)=2k*(2k+1) n!/2!(n-2)!=1*2*3*....*n/1*2*3*...*(n-2)*1*2=(n-1)n/2