let myFormR = document.querySelector(".myFormR");
let myFormD = document.querySelector(".myFormD");
let result = document.getElementById("resRad");
function calcRad(e) {
e.preventDefault();
let x = 2 * Math.PI * document.querySelector(".form-calcR").value;
result.innerHTML = x;
// alert(x);
}
function calcDiam() {
let z = Math.PI * document.querySelector(".form-calcD").value;
resRad.innerText = z;
}
let radOcr = myFormR.addEventListener("submit", calcRad);
let diamOcr = myFormR.addEventListener("submit", calcDiam);
<div class="calc container text-center">
Калькулятор полезных действий
<br />
<br />
<div>Длина окружности</div>
<form class="myFormR">
<input class="form-control form-calcR" type="text" placeholder="По радиусу" aria-label="default input example" />
</form>
<form class="myFormD">
<input class="form-control form-calcD" type="text" placeholder="По диаметру" aria-label="default input example" />
</form>
<div>Длина окружности равна</div>
<span id="resRad"></span>
</div>