y = √x, y' = 1/(2√x)
(1, 1), k = 1/2, 切线y - 1 = (1/2)(x - 1), y = (1/2)(x + 1)
法线斜率 = -1/k = -2, 法线: y - 1 = -2(x - 1), y = -2x + 3
