向量

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struct Vector
{
	double x, y;
	Vector() {}
	Vector(double a, double b) { x = a; y = b; }
	Vector operator-(Vector a) { return Vector(x - a.x, y - a.y); }
	Vector(Vector b, Vector a) { x = a.x - b.x; y = a.y - b.y; }
	Vector operator+(Vector a) { return Vector(x + a.x, y + a.y); }
	double Dot(Vector a) { return x * a.x + y * a.y; }
	double Cross(Vector a) { return x * a.y - y * a.x; }
	double Length() { return sqrt(x*x + y * y); }
};

圆

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struct circle
{
	Vector pt;
	double r;
	circle() {}
	circle(double x, double y, double z) { pt = Vector(x, y); r = z; }
	circle(int i) { scanf("%lf%lf%lf", &pt.x, &pt.y, &r); }
	inline double Area() {//面积
		return PI * r*r;
	}
	inline bool Intersect(circle C) {//相交
		double len = (C.pt - pt).Length();
		return Compare(len, (C.r + r)) < 0 && Compare(len, fabs(C.r - r)) > 0;
	}
	inline bool Inscribe(circle C) {//内切
		return Compare((C.pt - pt).Length(), fabs(C.r - r)) == 0;
	}
	inline bool External_Cutting(circle C) {//外切
		return Compare((C.pt - pt).Length(), fabs(C.r + r)) == 0;
	}
	inline double Perimeter() { return PI * 2 * r; }//周长
};