用泰勒公式做 lim x + √(x^2+x) = lim x + √[x^2(1+1/x)] = lim x + (-x)√(1+1/x) = lim x + (-x)[1+1/(2x) + o(1/x^2)] = lim x - x - 1/2 + o(1/x^2) = -1/2
