(1)设f(x)=ax+b,根据题意得:
2(a+b)+3(2a+b)=3①,2(-a+b)-b=-1②,
联立①②,解得:a=
4
9 ,b=-
1
9 ,
则 f(x)=
4
9 x-
1
9 ;
(2)设f(x)=ax 2+bx+c,根据题意得:
a(x+1) 2+b(x+1)+c+a(x-1) 2+b(x-1)+c=2x 2-4x,
化简得:(2a-2)x 2+(2b+4)x+2(a+c)=0,
即2a-2=0,2b+4=0,a+c=0,
解得:a=1,b=-2,c=-1,
则f(x)=x 2-2x-1.