1*5/1+5*9/1+9*13/1+13*17/1+17*21/1
问题描述：
1*5/1+5*9/1+9*13/1+13*17/1+17*21/1
答
1*5/1+5*9/1+9*13/1+13*17/1+17*21/1
=1/4(1-1/5)+1/4(1/5-1/9)+1/4(1/9-1/13)+1/4(1/13-1/17)+1/4(1/17-1/21)
=1/4[1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21]
=1/4(1-1/21)
=1/4×20/21
=5/21
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答
1/(1×5)＋1/(5×9)＋1/(9×13)＋1/(13×17)＋1/(17×21)
＝1/4×(1－1/5＋1/5－1/9＋1/9－1/13＋1/13－1/17＋1/17－1/21)
＝1/4×(1－1/21)
＝1/4×20/21
＝5/21
答
1*5/1+5*9/1+9*13/1+13*17/1+17*21/1=1/4(1-1/5)+1/4(1/5-1/9)+1/4(1/9-1/13)+1/4(1/13-1/17)+1/4(1/17-1/21)=1/4[1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21]=1/4(1-1/21)=1/4×20/21=5/21