4(2a²b-ab²)-5(ab²+2a²b)=(8a²b-4ab²)-(5ab²+10a²b)=8a²b-4ab²-5ab²-10a²b=(8-10)a²b+(-4-5)ab²=-2a²b-9ab² 将a=-1／2,b=1／3 代...