console 命令行工具 X clear
console
<!DOCTYPE html>
<html>
<body>
<div>圆的半径<input id="r" value=""></div>
<div>圆的周长<input id="cir" value=""></div>
<script>
	var r = Number(prompt("请输入半径："));
    var R = document.getElementById('r');
    var cir = document.getElementById('cir');
    var p = Math.PI；
    console.log(cir);
    if(r>=1 && r<=100){
    	R.value = r;
        cir.value = R*PI*2;
    }else{
        alert('请输入正确的值');
    }
    </script>
</body>
</html>