=1/165-1/165+1/157 +1/165+1/157 -1/157+1/165
=1/157+2/165| -1/165-1/157 |具体的步绪写一下| -1/165-1/157 |=| -(1/165+1/157) |=1/165+1/157 =1/165-1/165+1/157 -1/165-1/157 -1/157+1/165 =-1/157