function change_tab(tab_num) {
  hide_tabs();
		document.getElementById('tab_' + tab_num).style.display = "block";
		document.getElementById('tab_button_' + tab_num).className = "tab_on";
}

function hide_tabs() {
  document.getElementById('tab_1').style.display = "none";
  document.getElementById('tab_2').style.display = "none";
	document.getElementById('tab_3').style.display = "none";
		
	document.getElementById('tab_button_1').className = "tab_off";
	document.getElementById('tab_button_2').className = "tab_off";
	document.getElementById('tab_button_3').className = "tab_off";
}