kill(all)$

van=facebook.com/LearningCalculus$

b=sqrt((sqrt(2*R-a)*a*sqrt(a+2*R)*sqrt(2*R-c)*c*sqrt(c+2*R))/
       (2*R^2)-(a^2*c^2)/(2*R^2)+c^2+a^2);

b^2=rhs(%)^2;

second(solve(%,R));