ok... I figured out that my foci points : d=+/- sqrt(a) +sqrt(b)
Is this ok?

and how do I add this to my code below

thanks in advance

% Program to plot the hyperbola% y^2/a^2 - x^2/b^2 = 1% The hyperbolae are open up/down, so that x is the independent variable% for plotting. (Using the form x^2/a^2 - y^2/b^2 = 1 requires that y be% the independent variable, which is awkward programming-wise.)clear % all variablesfigure(1), hold off % start a new figureset(gca,'FontSize',14) % adjust fontsizexmax = 30; ymax = 20;x = linspace(-xmax,xmax,1001); % array of x values for plot : This function generates vector with 1001 points equally distributed between -xmax and xmax. %1001 is probably to have 500 points on one side (>0), 500 on another side (<0) and to include 0 itself. So you will have %more natural numbers. a = 5; b = 3;y=sqrt(((x.^2)./(b^2)+1).*a^2); % corresponding y valuesplot(x,y)hold on % add to current plotplot(x,-y) % Plot other half of hyperbolaaxis([-xmax xmax -ymax ymax]) % specify axis limitsxlabel('x')ylabel('y')title(['Hyperbola $y^2/a^2 - x^2/b^2 = 1$; $a$ = ', num2str(a), ...', $b$ = ', num2str(b),'; (WR 1/21/08)'],'Interpreter','latex')% Add axesplot([0 0],[-ymax ymax],'k') % y axis (black line - k)plot([-xmax xmax],[0 0],'k') % x axis% Program to plot the elipse% y^2/a^2 - x^2/b^2 = 1y=sqrt(((-x.^2)./(b^2)+1).*a^2); % corresponding y valuesplot(x,real(y))hold on % add to current plotplot(x,-y) % Plot other half of elipseaxis([-xmax xmax -ymax ymax]) % specify axis limitsxlabel('x')ylabel('y')title(['Hyperbola and elipse $y^2/a^2 + x^2/b^2 = 1$;$y^2/a^2 - x^2/b^2 = 1$; $a$ = ', num2str(a), ...', $b$ = ', num2str(b),';'],'Interpreter','latex')% Add axesplot([0 0],[-ymax ymax],'k') % y axis (black line - k)plot([-xmax xmax],[0 0],'k') % x axis

If c^2=a^2+b^2, then c=+-sqrt(a^2+b^2). Is it focus points?
I'd also add VerticalAlignment and HorizontalAlignment to make sure the pointers positions are closest.

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