HDU 2671 Can't be easier

简单的几何题目

点(a,b)关于直线Ax+By+C=1对称点的公式

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;

double k;
double X1,Y1,X2,Y2,X3,Y3,X4,Y4;
int T;
double a,b,c;

int main()
{
    scanf("%d",&T);
    while(T--)
    {
        scanf("%lf",&k);
        scanf("%lf%lf%lf%lf%lf%lf",&X1,&Y1,&X2,&Y2,&X3,&Y3);
        a=k,b=-1,c=Y3-k*X3;
        X4=X1-2*a*(a*X1+b*Y1+c)/(a*a+b*b);
        Y4=Y1-2*b*(a*X1+b*Y1+c)/(a*a+b*b);
        printf("%.2lf\n",max(sqrt((X2-X1)*(X2-X1)+(Y2-Y1)*(Y2-Y1)), sqrt((X2-X4)*(X2-X4)+(Y2-Y4)*(Y2-Y4))));
    }
    return 0;
}