kill(all);

van=facebook.com/LearningCalculus$

k*a=sqrt(a*(2*r-a))-r;

k*a-r = sqrt(a*(2*r-a));

lhs(%)^2-rhs(%)^2=0;

s:solve(%,r);

r1(a,k):=''(rhs(first(s)))$ r1(a,k);

r2(a,k):=''(rhs(second(s)))$ r2(a,k);

r1(4,2);

r2(4,2);