2a²b-3a-3a²b+2a
=(2a²b-3a²b)+(-3a+2a)
=-a²b-a
将a=-1/2 b=4代入
原式=-(-1/2)²×4-(-1/2)
=-1/4×4+1/2
=-1+1/2
=-1/2
答案：-1/2