MATLAB homework help

I've been asked to write a program that determines the center and radius of a circle that passes through three given points. The program should ask the user to enter the coordinates of the point one at a time. The program displays the coordinate of the center and the radius, and makes a plot of the circle and the three points displayed on the plot with asterisk markers.

I know how to do this algebraically with the distance formula yielding three equations and three unknowns but I'm new to using MATLAB (programming in general for that matter) and I'm having trouble translating that into a program using MATLAB.

Another way to do it is using the code below

x1=input('First x coordinate');
x2=input('Second x coordinate');
x3=input('Third x coordinate');
y1=input('First y coordinate');
y2=input('Second y coordinate');
y3=input('Third y coordinate');

t1 = x3^2-x2^2+y3^2-y2^2;
t2 = x1^2-x3^2+y1^2-y3^2;
t3 = x2^2-x1^2+y2^2-y1^2;
d = x1*y2-x2*y1+x2*y3-x3*y2+x3*y1-x1*y3
a = 1/2*(t1*y1+t2*y2+t3*y3)/d
b = -1/2*(t1*x1+t2*x2+t3*x3)/d

d is the radius, and the center of the circle is (a,b) but then plotting the circle is what I'm having trouble with.

I would really appreciate assistance Thanks!

Answer 1

[Daniel Shub]

This will plot a circle with radius 1 and center (0,0). Can you see where to add a and b to get a circle centered on (a,b)? How about scaling it to get diameter d?

x = -1:0.01:1;
plot([x, fliplr(x)], [sqrt(1-x.^2), -sqrt(1-fliplr(x).^2)]);
axis square;