简算:1/4×5+1/5×6+1/6×7..+1/39+40

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简算:1/4×5+1/5×6+1/6×7..+1/39+40
简算:1/4×5+1/5×6+1/6×7..+1/39+40

简算:1/4×5+1/5×6+1/6×7..+1/39+40
1/4×5+1/5×6+1/6×7..+1/39+40
=1/4-1/5+1/5-1/6+···+1/38-1/39+1/39-1/40
=1/4-1/40
=9/40