[R] Draw a perpendicular line?

[CZ]

Hi,

I am trying to draw a perpendicular line from a point to two points.
Mathematically I know how to do it, but to program it, I encounter some
problem and hope can get help.  Thanks. 

I have points, A, B and C.  I calculate the slope and intercept for line
drawn between A and B.
I am trying to check whether I can draw a perpendicular line from C to line
AB and get the x,y value for the point D at the intersection. 

Assume I get the slope of the perpendicular line, I will have my point (D)
using variable x and y which is potentially on line AB.   My idea was using
|AC|*|AC| = |AD|*|AD|+ |CD|*|CD|.  I don't know what function I may need to
call to calculate the values for point D (uniroot?).

Thank you. 



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