Question: What is "t~" and How to resolve a Warning message, Simplifying

Solved a "simple" dynamics problem involving the law of cosines... etc.  Part of the solution involves selecting the positive root from the resulting 2nd order equation in x(t)- below.  

eq1 := (r1 + r2)^2 = r1^2 + x(t)^2 - 2*r1*x(t)*cos(theta(t))  (law of cosines)

eq2:=diff(eq1,t)

eq3 := subs(diff(x(t), t) = v, diff(theta(t), t) = omega, eq2)  (remove xdot and theta dot)

assume(0 < x(t));
assume(0 < t);
assume(0 < theta(t) and theta(t) < 2*Pi);

xx := solve(eq3, v)  (creates an relationship between v, theta and x)

     xx := -r1*x(t~)*omega*sin(theta(t~))/(-cos(theta(t~))*r1 + x(t~))

Not clear what this t~ notation means?

when I solve eq1 above for x(t) there are two roots.  I select the positve root, but receive this warning...

eqx := solve(eq1, x(t))[1]

Warning, solve may be ignoring assumptions on the input variables.
        eqx := cos(theta(t))*r1 + sqrt(cos(theta(t))^2*r1^2 + 2*r1*r2 + r2^2)

How can I correct or address the warning message?

Finally,  to piece together the solution I perform this step

v := subs(x(t) = eqx, xx)

The result has a fairly obvious "simplification", but using "simplify" and "combine" only makes it worse...  Is there a combination of commands to accomplish a simplification.

Here is the entirety of the Maple...

restart;
with(Typesetting);
Settings(typesetdot = true);

eq1 := (r1 + r2)^2 = r1^2 + x(t)^2 - 2*r1*x(t)*cos(theta(t));
eq2 := diff(eq1, t);
eq3 := subs(diff(x(t), t) = v, diff(theta(t), t) = omega, eq2);
assume(0 < x(t));
assume(0 < t);
assume(0 < theta(t) and theta(t) < 2*Pi);
xx := solve(eq3, v);
eqx := solve(eq1, x(t))[1];
v := subs(x(t) = eqx, xx);

examine the result for v... easy to see the "simpificaion"... but not sure how to manipulate Maple to accomplish this...

JD