应该是加到90分之1
1/2+1/6+1/12+……+1/90
=1/1×2+1/2×3+1/3×4+……+1/9×10
=1-1/2+1/2-1/3+1/3-1/4+……+1/9-1/10
=1-1/10
=9/10