n√n/(n²-1)=√n³/(n²-1)==√(n³-n+n)/(n²-1)=√(n²-1)*n/(n²-1)+n/(n²-1)=√n+n/(n²-1)(n为自然数，且n>=2))
n根号[n/（n^2-1）]=根号[n^2*n/(n^2-1)]=根号『[n*（n^2-1）+n]/(n^2-1)』=根号[n+n/(n^2-1)]
