I want to show and hide markers by GoogleMap API zoom level

<script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?sensor=false"></script>
<script src="path for js below" charset="UTF-8"></script>
<divid="map_canvas-1"></div>

var currentInfoWindow=null;

function initialize() {
    var myLatlng = new google.maps.LatLng (34.4718128, 134.3301713);
    varmyOptions={

        zoom:16,
        center —myLatlng,
        mapTypeId: google.maps.MapTypeId.ROADMAP,

    };

    varmap = new google.maps.Map( document.getElementById("map_canvas-1"), myOptions);

// --------------------------- Marker 1 ---------------
var LatLng1 = new google.maps.LatLng (34.4703269, 134.3330037); // ★ Marker Location
var contentString1 = 
        '<div class="mapbox">test1</div>';//★info-window description
var infowindow1 = new google.maps.InfoWindow({
        content —ContentString 1
    });
  var image1 = new google.maps.MarkerImage('sample.png', // ★ marker type
      new google.maps.Size (55,72),
      new google.maps.Point(0,0),
      new google.maps.Point(0,32));
  varshadow1 = new google.maps.MarkerImage('http://maps.google.co.jp/mapfiles/ms/icons/tree.shadow.png',//★Marker Shadow
      new google.maps.Size (64,64),
      new google.maps.Point(0,0),
      new google.maps.Point(0,32));

varmarker1 = new google.maps.Marker({//★add marker)
        position:LatLng1,
        map —map,
        title: "test1",
        icon: image1,
        shadow —show1,
        animation —Google.maps.Animation.DROP
    });


// ★ Display only one info window
google.maps.event.addListener(marker1, 'click', function() {//★click action
if(currentInfoWindow){
currentInfoWindow.close();
}
infowindow1.open(map,marker1);
currentInfoWindow=infowindow1;
});


// ----------------------------- Marker 2 -------------
var LatLng2 = new google.maps.LatLng (34.4718128, 134.3301713);
var contentString2 = 
        '<div class="mapbox">test2</div>';   
var infowindow2 = new google.maps.InfoWindow({
        content —ContentString 2
    });
  varimage2 = new google.maps.MarkerImage('sample.png',
      new google.maps.Size (55,72),
      new google.maps.Point(0,0),
      new google.maps.Point(0,32));
  varshadow2 = new google.maps.MarkerImage('http://maps.google.co.jp/mapfiles/ms/icons/rail.shadow.png',
      new google.maps.Size (64,64),
      new google.maps.Point(0,0),
      new google.maps.Point(0,32));
varmarker2 = new google.maps.Marker({
        position:LatLng2,
        map —map,
        title: "test2",
        icon —image2,
        shadow —shadow2,
        animation —Google.maps.Animation.DROP
    });

// ★ Display only one info window
google.maps.event.addListener(marker2, 'click', function() {//★click action
if(currentInfoWindow){
currentInfoWindow.close();
}
infowindow2.open(map, marker2);
currentInfoWindow=infowindow2;
});


} // last