This seems like a homework problem. But Walter has already given an answer, that I think is not helpful, since dsolve will fail when applied directly. And since the problem is insufficiently defined to be a good homework problem, I might conjecture it is not in fact homework.
syms y(t) x(t) f(t)
dy = diff(y);
E1 = x + dy == f;
E2 = diff(x) + y == diff(f);
First, differentiate E1, subtracting E2 from that.
E3 = simplify(diff(E1) - E2)
As you can see, x(t) and f(t) have been completely eliminated from the result. However, now we can solve for y(t).
y(t) = dsolve(E3,y(0) == 0, dy(0) == 1)
In fact, if you think about it, note that this is a well known function : the hyperbolic sine function.
y = rewrite(y,'sinh')
Now that we have y(t), return to equations E1 and E2.
E1 = subs(E1)
From this we see that x(t) and f(t) are simply related, differening only by that cosh(t) term.
E2 = subs(E2)
The problem is, at this point you are now stuck. Lacking f(t) or y(t), you cannot find the other. What is worse, We see that now E1 and E2 are actually the same pieces of information, since we can derive E2 from E1. That is, just differentiate both sides of E1, and look to see what you get.
diff(E1)
SURPRISE! Differentiating E1 yields E2.
The point is, you lack sufficient information to solve the problem, even though you can solve for y(t). So perhaps that is the point of this assignment, IF it is one. Anyway, since the problem already had an answer, I might as well have written this.
