What error are you getting?
(perhaps you need "save carre,p,a,b,c,d,N,P,M ;")
A perpendicular line is also the line "rotated 90" (about the intersection
point).
Alan
On Sun, 26 Jan 2020 00:09:38 +0100
Fabrice Couvreur <fabrice1.couvr...@gmail.com> wrote:
> Hi,
> The point M is such that the line (CM) is perpendicular to the line (NP).
> I do not understand the error of this code.
> Thank you
> Fabrice
>
> \usecolors[xwi]
> \starttext
> \startMPcode
> path carre, p;
> carre = unitsquare scaled 138;
>
> pair a, b, c, d, N, P, M;
> a = point 0 of carre;
> b = point 1 of carre;
> c = point 2 of carre;
> d = point 3 of carre;
>
> p = b -- d;
> N = whatever[a,d];
> P = whatever[a,b];
> M = whatever[b,d];
> (M - c) dotprod (P - N) = 0;
>
> draw M -- c;
>
> draw p withcolor \MPcolor{navy};
> draw carre withcolor \MPcolor{firebrick};
>
> label.lft("$A$", point 0 of carre);
> label.lrt("$B$", point 1 of carre);
> label.urt("$C$", point 2 of carre);
> label.ulft("$D$", point 3 of carre);
> \stopMPcode
> \stoptext
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