areafcn.html: computes area of circle using function

<script>
    var rad;
    do {
    	rad = prompt("Enter radius")
        if (rad == "" || rad == null)
            break
        alert("radius = " + rad + ", area = " + area(rad))
    } while (rad != "" && rad != null)

    function area(r) {
    	return 3.141592654 * r * r
    }
</script>