(a+3)x^3-2x^2y+y^2-(5x^3+y^2+1)
=（a+3-5)x^3-2x^2y-1
不含x^3项,则有a+3-5=0
a=2
1/2(a^3-2a^2+4a-1)
=1/2(8-8+8-1)
=7/2