令y=f（x）y=(x²-3x+1)/(x+1)令t=x+1x>-1x+1>0t>0y=[(t-1)²-3(t-1)+1]/t=(t²-2t+1-3t+3+1)/t=(t²-5t+5)/t=(t+5/t)-5=[√t-√(5/t)]²+2√5-5t>0,[√t-√(5/t)]²≥0,y的最小值当√t=√(...