find values for the equation of a circle

Question

Duncan 2014-8-14

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https://ww2.mathworks.cn/matlabcentral/answers/147509-find-values-for-the-equation-of-a-circle

评论： Duncan 2014-8-25

采纳的回答： Roger Stafford

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I have the equation of a circle:

(x - a)^2 + (y - b)^2 - r^2 = 0

a, b and r are known values but I would like to find the values of x and y from a predetermined coordinate matrix which satisfies the equation.

example: a = 2, b = 2, r = 2
A = 
    1 2
    1 3
    2 4

hence the last row in A(2,4) would satisfy the equation. i.e. x = 2, y = 4

this is what I've tried so far:

a = 2;
b = 2;
r = 2;
x = find(A(:,1);
y = find(A(:,2);
solve((x - a)^2 + (y - b)^2 - r^2 == 0)

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Answer 1

Roger Stafford 2014-8-14

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https://ww2.mathworks.cn/matlabcentral/answers/147509-find-values-for-the-equation-of-a-circle#answer_148554

编辑：Roger Stafford 2014-8-14

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 a = 2; b = 2; r = 2;
 A = [1 2;1 3;2 4];
 B = A((A(:,1)-a).^2+(A(:,2)-b).^2==r^2,:);

B contains the coordinate pairs, if any, that lie on the circle. Note that if you need this for values of a, b, and c which are not integers, it is best to allow a small tolerance for round-off errors rather than demanding exact equality in the equation.

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Roger Stafford 2014-8-14

That was an error. I have now corrected it. My apologies.

Duncan 2014-8-25

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I have another question related to this. Say:

then the last two values would satisfy and hence display 2,4 and 4,2.

What I would like to know is how this code would change if i set limits. Say I want to display only the value that is within the limits of +/-1 of a & b.

i.e. it would only display 2,4

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