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计 算 题（数学·2020年·重庆市南开中学）
1/(4×5)+1/(5×6)+1/(6×7)+⋯+1/(39×40)
解答提示
原式=1/4-1/5+1/5-1/6+1/6-1/7+⋯+1/39-1/40
=1/4-1/40
=9/40
1/(4×5)+1/(5×6)+1/(6×7)+⋯+1/(39×40)
原式=1/4-1/5+1/5-1/6+1/6-1/7+⋯+1/39-1/40
=1/4-1/40
=9/40