6/7－6/7×﹙2/3＋1/9﹚
=6/7×﹙1-2/3-1/9﹚
=6/7×2/9
=4/21为什么括号里要变减减去两个数的和=分别减去这两个数。a-（b+c）=a-c-b5个1/5之间添符号，怎么得3(1/5+1/5)÷1/5+1/5÷1/5=3