Task 3

function run_demo() {
    var str = "Result=";
    var res = document.getElementById("result");
    var n = 3;
    var product = 1;
    for (var i = 1, p = 1, q = 2; i <= n; i++, p += 2, q += 2) {
        product *= (p / q);
    }

    res.innerHTML = str + product;
}