How to find the point(s) when two plot curves cross?

Question

tomer polsky 2018-7-1

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https://ww2.mathworks.cn/matlabcentral/answers/408281-how-to-find-the-point-s-when-two-plot-curves-cross

评论： Image Analyst 2018-7-1

Hello. I want to plot these 2 plots:

y_1=a*x+b;
y_2=a*(x^2);

and find to point where they meet? The exact values of x. This is my code so far, but I don't know how to finish the statement :

clc;
clear all;  
a=5;
b=10;
x=linspace(0,10,50);
y_1=a*x+b;
y_2=(x.^2);
for i=1:length(x);
    if(a*x+b==(x.^2));
        printf(x)
    end      
end
plot(y_1);
hold on;
plot(y_2);

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Answer 1

Image Analyst 2018-7-1

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/408281-how-to-find-the-point-s-when-two-plot-curves-cross#answer_327044

在 MATLAB Online 中打开

Use roots():

a=5;
b=10;
x=linspace(-4,10,50);
y_1=a*x+b;
y_2=(x.^2);
for i=1:length(x)
      if(a*x+b==(x.^2))
          printf(x)
    end
end
plot(x, y_1,'b-', 'LineWidth', 2);
hold on;
plot(x, y_2),'c-', 'LineWidth', 2;
grid on;
xRoots = roots([1, -a, -b])
y1 = a * xRoots(1) + b;
y2 = a * xRoots(2) + b;
plot(xRoots, [y1, y2],'ro', 'LineWidth', 2, 'MarkerSize', 13);

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tomer polsky 2018-7-1

ok lets so that matlab cant handle do equality with floating numbers . but if I represnt x as x=1:1:50 means that that there are not floating numbers .

and my question is there any way with out using roots command to find when y_1==y_2?

Image Analyst 2018-7-1

You can use ismembertol() like the FAQ says, but why? What's wrong with using roots? Why deny yourself the simplest, one-line solution? Usually people don't ask and prefer to do things the more complicated less accurate way so I'm not sure why you do.

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