How do I plot a function that has a value given

I'm trying to graph this question from here https://math.stackexchange.com/questions/2638189/express-density-as-a-ratio-of-height

I'm given values for D2 and D1, and then get the ratio D2/D1. I end up with an

equation 4*D2/D1 = 3x^2-x^3, and I need to acquire x from that equation and then plot it.

I tried doing the root of this equation, and when I tried to graph x, D2/D1, it says that "Vectors must be the same length".

Answer 1

[Star Strider]

Since you did not post your code, I cannot help you troubleshoot it.

Two options —

Symbolic:

syms x 
DRatio = 3*x^2-x^3;
xroot = solve(DRatio == 0)

xroot =
 0
 0
 3

Numeric:

xroots = roots([-1  3  0  0])

xroots =
     0
     0
     3

[jake stan]

a = [0.4, 0.87, 0.84,0.88] b = [0, 0, 0, 0]

for j = 1:4 
    r = roots([1 -3 0 4*a(j)]);
    b(j) = r(j);
end

plot(a,r);

EDITED: I just noticed that I'm suppose to put the roots into an empty array, and then plot the array A with array B, but I get multiple values for the cube root of my function, I'm not sure which one to put into the empty array

[Star Strider]

What is ‘[0.4, 0.87, 0.84,0.88]’?

[Walter Roberson]

I suspect the a values are 4*D2./D1

[jake stan]

Oh yeah sorry I just edited

[Star Strider]

Try this:

xroots = arrayfun(@(a) roots([1  -3  0  4*a]),  [0.4, 0.87, 0.84,0.88], 'Uni',0)
Roots = [xroots{:}]'

Roots =
    2.7952    0.8659   -0.6611
    2.3916    1.5483   -0.9398
    2.4318    1.4934   -0.9252
    2.3770    1.5676   -0.9446

Answer 2

[Walter Roberson]

For vector D1 and D2, you need D2./D1 instead of D2/D1