（1）3x^2+6x+x+2=3x^2+7x+2
(2) 4y^2-20y-y+5=4y^2-21y+5
(3)4/5x^2+xy-xy-5/4y^2=4/5x^2-5/4y^2
(4)(2a+b)^2=4a^2+4ab+b^2