Task 3

function run_demo() {
    var str = "Result=";
    var res = document.getElementById("result");
    var n = 7;
    var sum = 0;
    var x = 2.5;
    var rx = x;
    var factorial = 1; //1!
    var sign = 1;
    for (var i = 1; i <= n; i += 2) {
        var v = sign * rx / factorial;
        sum += v;
        sign = -sign;
        rx = rx * x * x;
        factorial = factorial * (i + 1) * (i + 2);
    }

    res.innerHTML = str + sum;
}