title: How can I get the distance between two point by latlng?
body: <p>I want to get the distance from A to B by latlng.When i run my app with the following code,it return a wrong result.
  Fx i tryed A( 31.172740,115.0081630) ,then B（30.6055980,114.3603140),the result is about 3111km.but the right distance is about 134km.
  My code is:</p>
  
  <pre><code>    class btnListener implements OnClickListener{
  
      public void onClick(View v) {
           lat_a=lat_A.getText().toString();
           lng_a=lng_A.getText().toString();
           lat_b=lat_B.getText().toString();
           lng_b=lng_B.getText().toString();
           double a1,n1,a2,n2;
           a1=30.6055980;
           a2=Double.parseDouble(lat_b);
           n2=Double.parseDouble(lng_b);
           double R=6371;
           double D=Math.acos(Math.sin(a1)*Math.sin(a2)+Math.cos(a1)*Math.cos(a2)*Math.cos(n2-n1));
           Toast.makeText(getApplication(), "the distance is"+String.valueOf(D*R)+"km from A to B", Toast.LENGTH_SHORT).show();
      }
  
  }
  </code></pre>