偶函数
所以f(x)=f(-x)
所以ax^2+(a+1)x+2=a(-x)^2+(a+1)(-x)+2
ax^2+(a+1)x+2=ax^2-(a+1)x+2
(a+1)x=-(a+1)x
2(a+1)x=0
所以a+1=0
a=-1
f(x)=-x^2+2
-2
偶函数
所以f(x)=f(-x)
所以ax^2+(a+1)x+2=a(-x)^2+(a+1)(-x)+2
ax^2+(a+1)x+2=ax^2-(a+1)x+2
(a+1)x=-(a+1)x
2(a+1)x=0
所以a+1=0
a=-1
f(x)=-x^2+2
-2