7-5+3= 8+2-6= 10-4+3=
5+2+3= 6+3-7= 9-5+2=
12+5= 8-6= 11+7=
17-5= 18-6= 18-7=
5+5= 7+7= 4+4= 8+8=
6+5= 7+8= 5+4= 8+9=
6+6= 8+8= 5+5= 9+9=
仔细观察,这几组题有什么特点?