(1/143+1/77+1/91)÷(20/99+1/9) - 2/91= [7/(11×13×7)+13/(77×13)+11/(91×11)]÷(20/99+11/99) - 2/91= 31/(11×13×7)÷31/(9×11) - 2/91= 1/(13×7×9) - 2/(13×7)= 1/(13×7×9) - 18/(13×7×9)= - 17/(1...你的思路是怎样的？(1/143+1/77+1/91)÷(20/99+1/9) - 2/91= [7/(11×13×7)+13/(77×13)+11/(91×11)]÷(20/99+11/99) - 2/91 【除号前后分别通分】= 31/(11×13×7)÷31/(9×11) - 2/91= 31/(11×13×7) × (9×11)/31 - 2/91【除法变乘法：除数分子变分母，分母变分子】= 9/(13×7)- 2/91= 9/91 - 2/91= 7/91