2/3a的2次方-6a的3次方+5a-1/4+5a的3次方-1/2-4/3a的2次方
=(2/3a的2次方-4/3a的2次方)+(5a的3次方-6a的3次方)+5a-1/4-1/2
=-2/3a的2次方-a的3次方+5a-3/4