Edit in JSFiddle

/* isで判定 */
$(function () {
	$('.accordionPanel').hide();
	$("input[name='accordion']:radio").change(function(){
	    if($("#ac02").is(':checked')){
	        $('.accordionPanel').slideDown();
	    }
	    else {
	        $('.accordionPanel').slideUp();                                       }
	});
});

<input type="radio" name="accordion" id="ac01" value="0" checked/>
<label for="ac01">radio1</label>
<input type="radio" name="accordion" id="ac02" value="1" />
<label for="ac02">ここをチェックで開く</label>
<input type="radio" name="accordion" id="ac03" value="2" />
<label for="ac03">radio3</label>
<div class="accordionPanel">
ここにテキストが入ります。
</div>