1/[(n+1)√n+n√(n+1)]
=[(n+1)√n+n√(n+1)]/[(n+1)^2*n-n^2*(n+1)]
=[(n+1)√n+n√(n+1)]/[n(n+1)]
=1/√n - 1/√(n+1)
所以结果为9/10