poj2242

正弦定理+叉积

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#include <iostream>

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <cmath>

using namespace std;



struct Point

{

    double x, y;

    Point operator -(const Point &b)

    {

        Point ret;

        ret.x = this->x - b.x;

        ret.y = this->y - b.y;

        return ret;

    }

}p1, p2, p0;



double xmult(Point a, Point b)

{

    return a.x * b.y - a.y * b.x;

}



double mol(Point a)

{

    return sqrt(a.x * a.x + a.y * a.y);

}



int main()

{

//    freopen("t.txt", "r", stdin);

    while (~scanf("%lf%lf%lf%lf%lf%lf", &p0.x, &p0.y, &p1.x, &p1.y, &p2.x, &p2.y))

    {

        Point a = p1 - p0;

        Point b = p2 - p0;

        Point c = p1 - p2;

        double R2 = abs(mol(c) / (xmult(a, b) / mol(a) / mol(b)));

        printf("%.2f\n", R2 * 3.141592653589793);

    }

    return 0;

}