restart;
eq1 := area = (Pi-(1/2)*theta)*rho^2-(Pi-(1/2)*theta)*rho0^2;

eq2 := (2*Pi-theta)*rho = 2*Pi*R;

eq3 := (2*Pi-theta)*rho0 = 2*Pi*r;

eq4 := rho0+dh = rho;

eq5 := (R-r)^2+dx^2 = dh^2;

S := solve([eq1, eq2, eq3, eq4, eq5], [area, theta, rho, rho0, dh]);

subs(S[1], area);