JavaScript求99乘法表+求梯形面积

 		var i,j;
        i = 0, j = 0;
        for (i = 1; i <= 9; i++) {
    
    
            for (j = 1; j <= i; j++) {
    
    
                document.write(j + "*" + i + "=" + i * j + "&nbsp;&nbsp");
            }
            document.write("<br/>");

        }
        var a, b, c;
        function sum(a, b, c) {
    
    
            var s = ((parseInt(a.value) + parseInt(b.value)) * (parseInt(c.value))) / 2;
            alert("梯形的面积是：" + s);
        }
    </script>
    <form>
        上底<input type="text" name="a"><br />
        下底<input type="text" name="b"><br />
        高度<input type="text" name="c"><br />
        <input type="button" onclick="sum(a,b,c)" value="求面积">