y=ax+b, y=cx+d (a≠0, c≠0)
If the two line cross perpendicularly ⇔ ac= -1
So a tangent of the perpendicular line is "-(1/5)".
Then, the equation of the perpendicular line is y=-(1/5)x+a (a is y-intercept).
On the other hand, this equation pass r(1, 3), so
3 = -(1/5) times 1 + a
∴ a=16 / 5
Therefore, y=-(1/5)x+16 /5 is answer.
By the way, I'm Japanese so if you find some mistakes in my English, please let me know.
