6分之1+12分之1+20分之1+30分之1+42分之1+90分之1
=1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)+1/(6×7)+1/(9×10)
=1/2-1/3+1/3-1/4+.+1/6-1/7+1/9-1/10
=1/2-1/7+1/9-1/10
=5/14+1/90
=116/315;
如果本题有什么不明白可以追问,