设极限式为A,1/x = -y,则y-inf,y->0A = [（x^2+3x+1) - (x^2-x+1) ]/ [ sqrt(x^2+3x+1) + sqrt(x^2-x+1)]分子分母同除以x,则A = (4 + y)/-[sqrt(1 - 3y + y^2) + sqrt(1 +y +y^2)]limA = -2