(1/143 + 1/77 + 1/91) / (20/99 + 1/9) - 2/91
= (1/143 + 1/77 + 1/91) / (31/99) - 2/91
= (9/13 + 9/7 + 99/91) / 31 - 2/91
= (63/91 + 117/91 + 99/91) / 31 - 2/91
= (279/91) / 31 - 2/91
= 9/91 - 2/91
= 7/91
= 1/13你的思路是怎样的？思路都体现在上面的各步计算中。就是通分、约分，使计算越往后越简明。