//dis:2地距离
//v_a:人走路的速度
//v_b:汽车速度
//必须：v_b >=v_a

float func(float dis,float v_a,float v_b)
{    
     float t1,t2;
     float c0=0,c1=dis;
     float k=(c0+c1)/2;//一开始就选2者中心距离。 
      
     for(;;){

       t1=k/v_b+                                            //甲开始走路时间 
          (k-k*v_a/v_b)/(v_a+v_b)+                          //和汽车相遇所需时间 
          (dis-(k/v_b+ (k-k*v_a/v_b)/(v_a+v_b))*v_a)/v_b;   //坐汽车走所需时间                     
       t2=k/v_b+(dis-k)/v_a;                                //乙,先汽车后走路 

 
       
       if(fabs(t1-t2)<0.00001)
       {
             printf("K:%f,t1:%f,t2:%f",k,t1,t2);   
             break;
       }else{           
              if(t1<t2)
              {
                    c0=k;
                   (k=(c1+k)/2);
              }else{
                     c1=k;
                    (k=(k+c0)/2);                    
              }      
       }
     }
     
     return k;
}