1/1×5+1/5×9+1/9×13+1/13×17+1/17×21简便算法

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1/1×5+1/5×9+1/9×13+1/13×17+1/17×21简便算法1/1×5+1/5×9+1/9×13+1/13×17+1/17×21简便算法1/1×5+1/5×9+1/9×13+1/13×

1/1×5+1/5×9+1/9×13+1/13×17+1/17×21简便算法
1/1×5+1/5×9+1/9×13+1/13×17+1/17×21简便算法

1/1×5+1/5×9+1/9×13+1/13×17+1/17×21简便算法
原式=1/1-1/5+1/5-1/9+1/9-1/13+1/13-1/17-1/21 =1/1-1/21 =20/21 答案是21分之20