MATLAB: Marking x intercepts on graph

MATLABplotting

Sorry everyone. I'm pretty new to matlab. My question was to graph an equation V against x for -4</= x </=4
This is the code I came up with:
x=linspace(-4,4);
V=(x.^3/3)-4*x;
plot(x,V)
xlabel('Position'),ylabel('Potential Energy')
I need to mark the x-intercepts on the graph. How do I go about doing that?

Best Answer

Method 1: solve equation for y=0
Use the Symbolic Math Toolbox to solve for y=0; see inline comments for details.
% Solve for y=0
syms x
eqn = x.^3/3-4*x == 0; 
xInt = double(solve(eqn)); % X values where y=0
yInt = zeros(size(xInt));  % Corresponding y values (all 0)
% plot function and x-intercepts
x=linspace(-4,4);
V=(x.^3/3)-4*x; 
plot(x,V,'k-')
hold on
plot(xInt,yInt, 'm*','MarkerSize', 10)
yline(0)
Method 2: use intersections() to find x-intercepts
This uses the intersections() function from the file exchange to find the (x,y) coordinates of the x-intercepts. 
x=linspace(-4,4);
V=(x.^3/3)-4*x;
[xInt,yInt] = intersections(x,V,x,zeros(size(V))); 
% ^^   ^^  There are your intercept coordinates
plot(x,V,'k-')
hold on
plot(xInt,yInt, 'm*','MarkerSize', 10)
yline(0)
Both methods produce this figure
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