draw the angle of perpendicular distance between point and a line

i tray with this coordinates, the point is R=(x0,y0),the line is AB where A=(x1,y1) and B=(x2,y2), but i didn't know if it is really perpendicular, and i can't draw the angle projection.

R = (2, 6) 
A = (8, 4)  
B = (2 ,1.8)

How can I choose the coordinates to get the perpendicular?

Write the (parameterised) equation of the straight line as

x=x1+t(x2-x1), y=y1+t(y2-y1)

Write the (squared) distance of point (x0,y0) from this line:

R^2=(x1+t(x2-x1)-x0)^2+(y1+t(y2-y1))^2

Differentiate R^2 with respect to t to find the minimum. (Line of code below).

Use this value of t to get the projection point. Note that if t lies outside [0,1] then the projection point is outside the line segment.

import math
import matplotlib.pyplot as plt

x0, y0 = 2, 6

x1, y1 = 8, 4
x2, y2 = 2, 1.8

t = -( (x2-x1)*(x1-x0)+(y2-y1)*(y1-y0) ) / ( (x2-x1)**2 + (y2-y1)**2 )
p, q = x1 + t * ( x2 - x1 ), y1 + t * ( y2 - y1 )

print( "Projection point is ", p, q )
print( "Nearest distance (to extended line) is ", math.sqrt( ( p - x0 ) ** 2 + ( q - y0 ) ** 2 ) )

plt.plot( [x0,x1,x2], [y0,y1,y2], 'o' )
plt.plot( [x1,x2], [y1,y2], '-' )
plt.plot( [x0,p ], [y0,q ], '--' )
plt.show()

Output:

Projection point is  3.357492654260529 2.2977473065621936
Nearest distance (to extended line) is  3.9432805267237567