12+23-34+45-56+67-78+89
把顺序换下: (89-78)+(67-56)+(45-34)+23+12=33+23+12=68
12+23-34+45-56+67-78+89
=12+45+89-78+(23+67)-(34+56)
=67+11
=78
12+23-34+45-56+67-78+89
=12+(23+67)-(34+56)+45-78+89
=12+45-78+89
=57+11
=68
45-34+67-56+89-78+12+23
=11+11+11+12+23
=68