# how to extend a straight line passing through two points until it meets the y axis of the Cartesian plane

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ELISABETTA BILLOTTA le 8 Oct 2022
Commenté : Star Strider le 8 Oct 2022
I have a graph of this type and I have created the straight line dashed in red that passes through the point of coordinates 0-0 and the point marked with the red dot. now I should extend this line until it meets the y axis, always maintaining the same inclination as the straight line. this is because then I need to identify the coordinates of the points of intersection between the straight line and the other curved lines with values 10 - 1 and 0.01.
how can I do? can anyone help me?
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dpb le 8 Oct 2022
Attach data and code so folks have something to work with...nothing can do with just the image...
Ghazwan le 8 Oct 2022
It looks like you are doing this manually. It is way better to make a code for this line. With the code, we can make an equation for and find the intersection points.

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### Réponse acceptée

Star Strider le 8 Oct 2022
It already meets the y-axis at the origin (0,0).
If you want to extend it to , that is straightforward —
slope = -53634 / -234311;
intercept = 0;
yfit = -5E+5*slope + intercept
yfit = -1.1445e+05
figure
plot([-5E+5 0], [yfit 0], '--r')
grid
axis([-5E+5 1E+5 -3.1E+5 2.5E+5])
I do not have your contourf plot, so I cannot plot it with that as well.
.
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ELISABETTA BILLOTTA le 8 Oct 2022
thank you!!! works!!!
Star Strider le 8 Oct 2022
As always, my pleasure!

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### Plus de réponses (1)

Image Analyst le 8 Oct 2022
Is this like what you want?
x = [1, 5];
y = [2, 7];
plot(x, y, 'b.-', 'LineWidth', 2, 'MarkerSize', 30)
grid on;
xlim([0, 6])
ylim([0, 8])
% Fit a line through x and y
coefficients = polyfit(x, y, 1);
% Get the value on the y axis, which is when x=0.
x0 = 0;
y0 = polyval(coefficients, x0)
y0 = 0.7500
% Plot a line from the y axis to the point [x1, y1]
hold on;
plot([x0, x(1)], [y0, y(1)], 'r-', 'LineWidth', 2)
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