JSFiddle - React, Tailwind, and code Playground

HTML

<input id="A" type="text" placeholder="a" value="1"><br>
<input id="H" type="text" placeholder="h" value="2"><br>
<input id="N" type="text" placeholder="n" value="3"><br>
<div id="result"></div>

JavaScript

function f (x) {
     return (x * x + 1) * Math.pow(Math.cos(x), 2);
}

function F(y, a, h, n) {
    return n < 1 ? f(a) + y : F(2 * f(a + n*h), a, h, n-1) + y;
}

function updateResult () {
    var a = A.value;
    var h = H.value;
    var n = N.value;
    result.innerHTML = F(f(a + n*h), a, h, n-1);
}

$('input').keyup(updateResult);

updateResult();