HDU2080 夹角有多大II

余弦定理的运用，利用原点和题目给定两点构造三角形。

算出三边长度即可利用反余弦函数求得角的大小。

/*************************************************************************
    > File Name: 2080.cpp
    > Author: Wallance
    > Mail: halyx@126.com 
    > Created Time: Thu 29 Jan 2015 10:08:55 PM HKT
 ************************************************************************/

#include <iostream>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <list>
#include <set>
#include <stack>
#include <map>
#include <queue>
#include <string>
#include <vector>
#include <cctype>
using namespace std;

int main(int argc, char const* argv[])
{
	int ncase;
	double x1,x2,y1,y2;
	scanf("%d", &ncase);
	while (ncase--)
	{
		scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2);
		double d1,d2,d3;
		d1 = x1*x1+y1*y1;
		d2 = x2*x2+y2*y2;
		d3 = (x2-x1)*(x2-x1)+(y2-y1)*(y2-y1);
		double ans;
		ans = acos((d1+d2-d3)/(2*sqrt(d1)*sqrt(d2)))/(4*atan(1.0))*180;
		printf("%.2lf\n", ans);
	}
	return 0;
}