How to find the value of x when y = 0 and label on the curve?

Hello,

I'm trying to find and label the coordinate of (x,y) when y=0 but I don't succeed.

x = 0:0.01:66.03;

y=(3.7*(10*(x/66.03).^3-15*(x/66.03).^4+6*(x/66.03).^5)-1.86);

plot(x,y,'c-','LineWidth',3);

Thank you in advance for you help...

Answer 1

[Star Strider]

Using interp1 —

x = 0:0.01:66.03;
y=(3.7*(10*(x/66.03).^3-15*(x/66.03).^4+6*(x/66.03).^5)-1.86);

x_at_y0 = interp1(y,x,0)

x_at_y0 = 33.1102

figure
plot(x,y,'c-','LineWidth',3);
hold on
plot(x_at_y0, 0, 'r+', 'MarkerSize',15)
hold off
text(x_at_y0, 0, sprintf('  \\leftarrow (%.2f, %.2f)',x_at_y0,0), 'Horiz','left', 'Vert','middle')

.

[LIM MING HUI]

Thank you very much!!

[Star Strider]

As always, my pleasure!

Answer 2

[KSSV]

x = 0:0.01:66.03;
y=(3.7*(10*(x/66.03).^3-15*(x/66.03).^4+6*(x/66.03).^5)-1.86);
[val,idx] = min(abs(y)) ;

plot(x,y,'c-','LineWidth',3);
hold on
plot(x(idx),y(idx),'*r')

[LIM MING HUI]

But this show the y=-0.00993621... My target is to get x when y=0...

[KSSV]

fun  = @(x) (3.7*(10*(x/66.03).^3-15*(x/66.03).^4+6*(x/66.03).^5)-1.86);
x0 = fzero(@(x) fun(x), 33)

x0 = 33.1102

fun(x0)

ans = 2.2204e-16