kill(all)$

van=facebook.com/LearningCalculus$

%pi/4-3/8;

float(%);

distance(A,B):=sqrt((A[1]-B[1])^2+(A[2]-B[2])^2);

c(h,k,r):=(x-h)^2+(y-k)^2 - r^2;

C(h,k,r):=expand(c(h,k,r));

Q(A,B,C):=x^2+y^2+A*x+B*y+C;

P:Q(A,B,C)=C(h,k,r);

C(h,k,r);

[A,B,C];ABC:[rhs(coeff(P,x)),rhs(coeff(P,y)),pending];

ABC[3]:C(h,k,r)-x^2-y^2-ABC[1]*x-ABC[2]*y$ [A,B,C];ABC;

P1:[   0,   1]$
P2:[ 1/2, 1/2]$
P3:[ 1/2,   0]$
Pc:[-1/4, 1/4]$

r:distance(P1,Pc);
h:Pc[1]$
k:Pc[2]$

C(h,k,r);

float(r);