f(x+1)=X方-3x令t=x+1,则x=t-1则f(t)=(t-1)²-3(t-1)=t²-2t+1-3t+3=t²-5t+4所以f(x)=x²-5x+4f(x-1)=(x-1)²-5(x-1)+4=x²-2x+1-5x+5+4=x²-7x+10