设 f(x)=ax^2+bx+c
f(x)+2f(-x)=ax^2+bx+c+(2ax^2-2bx+2c)
=3ax^2-bx+3c
= x^2-2x+3
3a=1 -b=-2 3c=3
a=1/3 b=2 c=1
f(x)=(1/3)x^2+2x+1
设 f(x)=ax^2+bx+c
f(x)+2f(-x)=ax^2+bx+c+(2ax^2-2bx+2c)
=3ax^2-bx+3c
= x^2-2x+3
3a=1 -b=-2 3c=3
a=1/3 b=2 c=1
f(x)=(1/3)x^2+2x+1