HD2080

算夹角，水题一道。。。

#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

const double pi=3.1415926;

int main(int argc, char *argv[]) {
	int n;
	scanf("%d",&n);
	while(n--)
	{
		double x1,x2,y1,y2,t,a,b,c,max;
		scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
		a=sqrt(x1*x1+y1*y1);
		b=sqrt(x2*x2+y2*y2);
		c=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
		t=(a*a+b*b-c*c)/(2.0*b*a);
		t=acos(t)*180/pi;
		while(t>180)
		t-=180;
		printf("%.2lf\n",t);
	}
	return 0;
}