Matlab：使用solve或fsolve后退euler，不工作

Question

这应该是一个倒退的欧拉绘图仪，但它不起作用：

Warning: Explicit solution could not be found. 
> In solve at 83
  In u1cBAEUL at 29
???  In an assignment  A(I) = B, the number of elements
in B and
 I must be the same.

Error in ==> u1cBAEUL at 45
    q1(i+1) = a;

我使用solve：

clear all, close all, clc

stl= 0.05; % steplength
nos = 5000; % number of steps

q1 = zeros(1,nos);
q2 = zeros(1,nos);
q1v = zeros(1,nos);
q2v = zeros(1,nos);
q1a = zeros(1,nos);
q2a = zeros(1,nos);

% Starting positions
a=0.5;
q1(1) = 1-a;
q2(1) = 0;
q1v(1) = 0;
q2v(1) = sqrt((1+a)/(1-a));

% P is inertia, and has the same vector components as the body velocity

for i=1:nos-1    

    [q1a(i),q2a(i)] = u1FUNC(q1(i),q2(i));

    q1v(i+1) = q1v(i) + q1a(i).*stl; 
    q2v(i+1) = q2v(i) + q2a(i).*stl;

    [a,b] = solve ('a = q1(i) + stl*((q1v(i+1) + stl*(u1FUNC(a,b))))','b = q2(i) + stl*((q2v(i+1) + stl*(u1FUNC(a,b))))');
    % q1'' = v1'
    % v1 = f(q1,q2)
    %
    % q1n+1 = q1n + stl*v1n+1
    % v1n+1 = v1n + stl*f(q1n+1,q2n+1)
    %
    % q2'' = v2'
    % v2 = g(q1,q2)
    %
    % q2n+1 = q2n + stl*v2n+1
    % v2n+1 = v2n + stl*g(q1n+1,q2n+1)
    %
    % q1n+1 = q1n + stl(v1n + stl*f(q1n+1,q2n+1)
    % q2n+1 = q2n + stl(v2n + stl*g(q1n+1,q2n+1)

    q1(i+1) = a;
    q2(i+1) = b; 

end

我应该使用fsolve代替solve吗？