原式=3a²b-[2a²b-(2abc-a²c-4a²c)]-2abc
=3a²b-(2a²b-2abc+a²c+4a²c)-2abc
=3a²b-2a²b+2abc-a²c-4a²c-2abc
=(3a²b-2a²b)+(2abc-2abc)+(-a²c-4a²c)
=a²b-5a²c
当a=0.5, b=70, c=10时
原式=0.5²×70-5×0.5²×10
=0.25×70-5×0.25×10
=17.5-12.5
=5