Draw a line 90 degrees to other line

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Hi,

Maybe you can help me. Is there a function or a easy way to draw a line 90 degrees from a other line. I already have the startpoint but I dont know if its possible to set the degrees in the plot function. This is my triangle:

A = [0,0]
B = [6,3]
C = [1,4]
A = vector(A)
B = vector(B)
C = vector(C)
def triangle(A,B,C):
    g = line([A,B])
    g = g + line([B,C])
    g = g + line([C,A])
return g
startpoint= (A + B)/2
f = triangle(A,B,C)
plot(f)

Thank you