# How to plot a (which is changing from 0-1 in 0.01 increments) vs x(2) (using a for loop and fsolve to find the solution of a nonlinear equation containing x(s) sol based on a

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Victor Jimenez Carrillo on 16 Sep 2021
Commented: Alan Stevens on 16 Sep 2021
V=1000; Q=50; Ca0=1; k=1;
for a=0:1:0.1
f=@(x) [Q*Ca0-Q*x(1)-k*x(1)^2*(a*V); Q*x(1)-Q*x(2)-k*x(2)^2*(1-a)*V];
fsolve(f,[0.5,0.5])
end
plot(a,x(2))

Alan Stevens on 16 Sep 2021
Your first equation is a simple quadratic in x(1); your second is a quadratic in x(2) that depends on x(1), so, assuming you are only interested in the positive roots, these can be solved as follows:
V=1000; Q=50; Ca0=1; k=1;
a = 0:0.01:1;
x1 = zeros(1,numel(a));
x2 = zeros(1,numel(a));
for i=1:numel(a)
% assuming you want positive values of x1 and x2
if a(i) == 0
A = k*V;
x1(i) = Ca0;
x2(i) = (-Q + sqrt(Q^2 + 4*A*Q*x1(i)))/(2*A);
elseif a(i) == 1
A = k*V;
x1(i) = (-Q + sqrt(Q^2 + 4*A*Q*Ca0))/(2*A);
x2(i) = x1(i);
else
A1 = k*a(i)*V;
A2 = k*(1-a(i))*V;
x1(i) = (-Q + sqrt(Q^2 + 4*A1*Q*Ca0))/(2*A1);
x2(i) = (-Q + sqrt(Q^2 + 4*A2*Q*x1(i)))/(2*A2);
end
end
subplot(2,1,1)
plot(a,x1),grid
xlabel('a'),ylabel('x1')
subplot(2,1,2)
plot(a,x2),grid
xlabel('a'),ylabel('x2') % The two equations can be expressed as:
% k*a*V*x1^2 + Q*x1 - Q*Ca0 = 0
% k*(1-a)*V*x2^2 + Q*x2 - Q*x1 = 0
##### 2 CommentsShowHide 1 older comment
Alan Stevens on 16 Sep 2021
Your equations would then be cubic polynomials. Look up help on “roots” to see how to find the values.

R2020b

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