分数乘法的简便计算1/6+ 1/12+1/20 +1/30 +1/42 + 1/56+1/72怎么解答,怎么归类这样的题目?

问题描述:

分数乘法的简便计算
1/6+ 1/12+1/20 +1/30 +1/42 + 1/56+1/72怎么解答,怎么归类这样的题目?

1/6+ 1/12+1/20 +1/30 +1/42 + 1/56+1/72
=1/2*3+1/+1/3*4+1/4*5...1/8*9
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)..-1/9
=1/2-1/9=7/18
公式1/n(n+a)=1/a×(1/n-1/(n+a) )