(1)使结果不含常数项
(1/2a²+a-4)+(1/2a²+5a+4)
=1/2a²+a-4+1/2a²+5a+4
=a²+6a
(2)使结果不含二次项
(1/2a²+5a+4)-(1/2a²-a)
=1/2a²+5a+4-1/2a²+a
=6a+4