Vector: r⃗=a⃗+λb⃗\vec r = \vec a + \lambda \vec br=a+λb. Cartesian: x−x1a=y−y1b=z−z1c\dfrac{x - x_1}{a} = \dfrac{y - y_1}{b} = \dfrac{z - z_1}{c}ax−x1=by−y1=cz−z1.