f(2x+1)=x2-2x
令y=2x+1
x=(y-1)/2
f(y)=[(y-1)/2]²-(y-1)
=y²/4-3y/2+5/4
所以,f(x)=x²/4+3x/2+5/4
f(3)=f(2*1+1)=1²-2*1=-1