fn(x)的导数为n^2*X*(1-X)^(n-1)*(2-nX-2X)当X=0,1,2/(n+2)时导数为0将这三个值分别带入fn(x)中可得0,0,4(n/n+2)^(n+1)所以最大值为4(n/n+2)^(n+1)