向量法求 T3 这个若智 r^2

int sqr_vector_dis(node a){
    return a.x*a.x+a.y*a.y;
}
frac vector_multi(node a,node b){
    return frac(a.x*b.x+a.y*b.y,1);
}
frac dist(node a,node b,node c){
    //distance from c to line ab
    node vector1={b.x-a.x,b.y-a.y};
    node vector2={b.x-c.x,b.y-c.y};
    frac ans=vector_multi(vector1,vector2)*vector_multi(vector1,vector2);
    ans=ans/frac(sqr_vector_dis(vector1),1);
    ans=ans.abs();ans=ans/4;
    return ans;
}