（因为求凸包时允许了三点共线的存在，所以有-1.0）
angle = acos(((a.x-b.x)*(c.x-b.x) + (a.y-b.y)*(c.y-b.y)) / dist(a, b) / dist(b, c));
((a.x-b.x)*(c.x-b.x) + (a.y-b.y)*(c.y-b.y)) / dist(a, b) / dist(b, c) 略小于-1.0


之后改成了
db angle(pii a, pii b, pii c){
	db tmp = ((a.x-b.x)*(c.x-b.x) + (a.y-b.y)*(c.y-b.y)) / dist(a, b) / dist(b, c);
	if(tmp < -1.0) tmp = -1.0;
	if(tmp > 1.0) tmp = 1.0;
	return acos(tmp);
}