y = 2x + 3的斜率是2
即tanθ = 2 ==> θ = arctan(2)
直线L的倾斜角是2arctan(2)
斜率 = tan[2arctan(2)]
= 2tan(arctan2)/[1 - tan²(arctan2)]
= 2(2)/[1 - 2²]
= 4/(- 3)
= - 4/3
所以直线L的方程为y - 4 = (- 4/3)(x - 1)
即
4x + 3y - 16 = 0
公式：tan(2A) = 2tanA/(1 - tan²A)