# How to intersect these lines?

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Emmanuel on 29 May 2014
Commented: Emmanuel on 29 May 2014
Hi all! Given below is the code for plottong three lines given any two arbitrary points per line. I have to find the vertices of the intersection points of these lines. I am getting the plot , but they are not intersecting. I have attached the image below, please help!!
%For Line 1
[x1] = input('Enter the value of 1st coord');
[y1] = input('Enter the value of 1st coord');
t = linspace(0,1);
xa = x1(1)+ (y1(1)-x1(1))*t;
ya = y1(1) + (y1(2)-x1(1))*t;
%For Line 2
[x2] = input('Enter the value of 1st coord');
[y2] = input('Enter the value of 1st coord');
u = linspace(0,1);
xb = x2(1)+ (y2(1)-x2(1))*u;
yb = y2(1) + (y2(2)-x2(1))*u;
%For Line 3
[x3] = input('Enter the value of 1st coord');
[y3] = input('Enter the value of 1st coord');
v = linspace(0,1);
xc = x3(1)+(y3(1)-x3(1))*v;
yc = y3(1)+(y3(2)-x3(1))*v;
plot(xa,ya,xb,yb,xc,yc);

David Sanchez on 29 May 2014
I give you the code for two lines, you extend it to three (do it by pairs
if true
% code
end):
%For Line 1
[x1] = [2 3];%input('Enter the value of 1st coord');
[y1] = [3 4];%input('Enter the value of 1st coord');
ma = (y1(2)-y1(1))/(x1(2)-x1(1));
na = y1(1) - ma*x1(1);
%For Line 2
[x2] = [2 4];%input('Enter the value of 1st coord');
[y2] = [2 -3];%input('Enter the value of 1st coord');
mb = (y2(2)-y2(1))/(x2(2)-x2(1));
nb = y2(1) - mb*x2(1);
syms x
solve(x*ma + na == x*mb + nb)
ans =
12/7
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Emmanuel on 29 May 2014
for i=1:1:3
[A] = input('enter the 1st co-ordinate of the line');
[B] = input('enter the 2nd co-ordinate of the line');
xlim = [-10 10];
m = (B(2)-B(1))/(A(2)-A(1));
n = B(2)*m - A(2);
y1 = m*xlim(1) + n;
y2 = m*xlim(2) + n;
hold on
line([xlim(1) xlim(2)],[y1 y2])
plot(y1,y2)
hold off
end
%SAMPLE OUTPUT
%enter the 1st co-ordinate of the line[1,5]
%enter the 2nd co-ordinate of the line[3,2.5]
%enter the 1st co-ordinate of the line[3,1]
%enter the 2nd co-ordinate of the line[4,1]
%enter the 1st co-ordinate of the line[4,1]
%enter the 2nd co-ordinate of the line[4.5,5.8]
When I plot this on a paper and the output graph by matlab varies a lot. if i change the xlim to [0 20] , the lines do not intersect at all. Why does the output from matlab varies from that of the paper graph even if I use the same scale? Ihave attached the output plot. Can we like rectify it? 