答：
(2n)!
=1*2*3*...*(2n-1)*2n
=1*3*5*...*(2n-1)*2*4*6*...*2n
=1*3*5*...*(2n-1)*2^n*(1*2*3*..*n)
=1*3*5*...*(2n-1)*2^n*n!
所以(2n)!/(2^n*n!)
=1*3*5*...*(2n-1)*2^n*n!/(2^n*n!)
=1*3*5*...*(2n-1)