.3ab-4ab+8ab-7ab+ab=______． 2．7x-(5x-5y)-y=______． 3．23a3bc2-15ab2c+8abc-24a3bc2-8abc=______． 4．-7x2+6x+13x2-4x-5x2=______． 5．2y+(-2y+5)-(3y+2)＝______． 6．(2x2-3xy+4y2)+(x2+2xy-3y2)=______．...