1/2×3+1/3×4+……+1/19×20简便计算?
问题描述：
1/2×3+1/3×4+……+1/19×20简便计算?
答
1/(2×3)+1/(3×4)+……+1/(19×20)
=(1/2-1/3)+(1/3-1/4)+……+(1/19-1/20)
=1/2-1/20=9/20