已知x+y=3,求代数式3（x+y）²-4/x+y+2x-3+2y的值.3（x+y）²-4/x+y+2x-3+2y=3(x+y)²-4/(x+y)+2(x+y)-3=27-4/3+6-3=30-4/3=28又 2/3若二次多项式a²+2a+m中以2代替a,则代数式的值为0,求m的值2²...